| 3 SEASONAL PATTERNS OF TWO FOREST SOIL QUALITY INDICATORS: MICROBIAL BIOMASS AND NET N MINERALIZATION RATES - ÉTUDE DES PATRONS DE VARIABILITÉ SAISONNIÈRE DE DEUX INDICATEURS DE LA QUALITÉ DES SOLS : LA BIOMASSE MICROBIENNE ET LA CINÉTIQUE DE MINÉRALISATION NETTE DE L’AZOTE | ||
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Table des matières
In a context of monitoring impacts of silvicultural treatments on soil quality it is very important to better understand the seasonal patterns of indicators studied. In this study we evaluated the impacts of fertilization, herbicide application and scarification, applied alone or combined, on seasonal patterns of microbial biomass C, microbial biomass N, and net N mineralization rate. In situ buried bags were used to estimate net N mineralization at approximately 6-week intervals during the growing season. Microbial biomass C and N concentrations were also determined four times over spring to autumn of the same year. Both organic layer and mineral soil showed strong temporal variations. Even if seasonal patterns differed among indicators they were always more pronounced in the organic layer than in the mineral soil. Throughout the season, the variation of microbial biomass N was related to soil water content, whereas microbial biomass C was related to soil moisture only during August, the driest month of the season. We also noted that variability of all properties of both organic layer and mineral soil was high at each sampling date and that results varied highly throughout the season. In order for these indicators to be useful in the evaluation of the impacts of the silvicultural treatments on the soil quality, we recommend 1) to measure only the organic layer properties; and 2) to take measurements consistently at the same time period during the growing season.
Dans cette étude nous avons évalué l’impact de la fertilisation et de la pulvérisation d’herbicide, appliqués séparément ou combinés, sur les patrons de variabilité saisonnière de deux indicateurs de la qualité des sols : la biomasse microbienne et la minéralisation de l’azote. Tout au long de la saison de croissance, la méthode des sachets enfouis a servi à estimer la minéralisation de l’azote tandis que la biomasse microbienne a été estimée en utilisant la méthode de fumigation au chloroforme. Ces indicateurs présentent tous deux une grande variabilité saisonnière et ce, autant dans le sol minéral que dans l’horizon organique. Cependant la variabilité saisonnière est beaucoup plus marquée dans la couche organique que dans le sol minéral. La biomasse microbienne N est toujours corrélée au taux d’humidité alors que la biomasse microbienne C ne l’est qu’au mois d’août, moment où le stress hydrique est le plus important. Nous avons également noté que la variabilité saisonnière est très importante et que l’impact des traitements sylvicoles varie au cours de la saison. Aussi, afin d’utiliser au mieux ces indicateurs dans un but de suivi au cours du temps de l’impact des traitements sur la santé des sols, nous recommandons 1) de ne les mesurer que dans la couche organique des sols et 2) de toujours les mesurer à la même période.
Soil is the productive basis for forest ecosystem production, and the interaction of soil properties and biogeochemical cycles with biotic communities influences ecosystem structure and biodiversity at different temporal and spatial scales (Beare et al. 1995). The Canadian Council of Forest Ministers developed a framework of criteria and indicators to guide the sustainable development of forests in Canada (CCMF 1995), that includes criteria and indicators related to soil conservation and maintenance of ecosystem productivity. The applicability of this framework through certification depends on many factors, including the effectiveness of indicators at providing insight into the long term effects of anthropogenic activities on forest ecosystems (Staddon et al. 1999).
Maintenance of the quantity and quality of soil organic matter is imperative to maintain forest health (O'Laughlin et al. 1994) and productivity (Nambiar 1996, Powers et al. 1998). Several authors have proposed microbial biomass C (MBC), microbial biomass N (MBN), and net N mineralization rate as soil quality indicators (Harris and Bezdicek 1992, Visser and Parkinson 1992, Doran and Parkin 1994, Gregorich et al. 1994, Bauhus and Khanna 1999, Burger et al. 1999, Staddon et al. 1999). Soil microbial biomass is the principal component of the decomposer subsystem regulating nutrient cycling, energy flow and finally, plant and ecosystem productivity. Microbial biomass pools and activities are important ecosystem characteristics to be used for predicting rates of nutrient cycling. Soil microbial biomass is not a static entity and its temporal dynamics are likely to be extremely important in determining the extent of release of immobilized labile nutrients (e.g. nitrogen, phosphorus), and hence the availability of these nutrients for other components of the ecosystem (Diaz-Raviña et al. 1993, Bauhus and Barthel 1995). The passage of seasons may influence microbial numbers and mass either directly, through changes in microclimate (Sato and Seto 1999, Verburg et al. 1999), or indirectly, by influencing plant metabolism that feeds back to the soil ecosystem (Anderson and Domsch 1980, Ladd et al. 1985). Nitrogen mineralization rates are intimately connected to microbial biomass, responding to the same seasonal changes in factors controlling microbial activity : soil temperature, soil moisture, and organic C (Corg) availability. Soil microbial biomass pools and activity may increase in response to soil warming (Paul and Clark 1989, Sarathcandra et al. 1989), however, increased temperature may also provoke a decrease in soil moisture, reducing microbial biomass and activity (van Gestel et al. 1993). An increase in the microbial biomass pool can lead to nutrient immobilization (Holmes and Zak 1994) and therefore decrease the availability of nutrients for other ecosystem components. However, an increase in microbial activity can lead to increased net N mineralization and therefore an increase in nutrient availability to plants. In temperate ecosystems similar to that of the present study, soil moisture is more frequently limiting than temperature for microbial biomass during the growing season (Gonçalves and Carlyle 1994, Arnold et al. 1999) and N mineralization (Barg and Edmonds 1999) than is soil temperature. Barg and Edmonds (1999) reported no significant correlation between soil temperature and microbial biomass or between soil temperature and total N mineralization rates. However, in their study, both microbial biomass (r = 0.66) and total N mineralization rates (r = 0.78) were positively correlated with soil moisture. In a temperate poplar stand, Stoyan et al. (2000) also found a positive correlation (r = 0.40) between soil respiration and soil moisture.
Most studies of seasonal fluctuations of microbial biomass have been made on agricultural soils, and the results are not easily generalizable. Some authors found large seasonal fluctuations in microbial biomass and in net N mineralization (Ross et al. 1981), while others observed only minor seasonal changes (Schnürer et al. 1986, Patra et al. 1990). Holmes and Zak (1994) demonstrated that seasonal patterns of microbial biomass and net N mineralization rate may differ. They reported no significant seasonal fluctuations in MBC and MBN, whereas net N mineralization rate showed a marked seasonal pattern.
Because the indicators proposed are affected by microclimatic conditions and Corg availability, we supposed that they would present seasonal patterns. Since N availability is directly controlled by microbial activity, we hypothesized that microbial biomass and net N mineralization are inversely related on a seasonal basis. We also hypothesized that we would discern a temporal relationship between microbial biomass and soil moisture, since this factor seems to exert a predominant control on these factors. In a recent article we illustrated that microbial biomass and net N mineralization are still affected by silvicultural treatments carried out on this site ten years before (Périé and Munson, 2000). Hence, we were interested in the influence of treatments on specific seasonal patterns of indicators, and the potential interpretation of ecosystem function as a result of these patterns. To test these hypotheses, we studied i) the seasonal patterns of these three soil quality indicators; ii) the temporal relationship between soil moisture, microbial biomass, and net N mineralization; and iii) the influence of silvicultural treatments on the seasonal patterns of indicators.
The experimental site is situated in central Ontario (Canada), on the north shore of Cartier Lake (45°57’50’’ N, 77°34’45’’ E; 170 m above sea level) within the Petawawa Research Forest. It is in the Middle Ottawa Section of the Great Lakes-St. Lawrence Forest region (Rowe 1984). The underlying bedrock is Precambrian granite and gneiss, and the soil is a deep, well-drained loam to sandy-loam, classified as an orthic humoferric podzol (Soil Classification Working Group, 1998). The regional climate is moist-humid (Hills 1959), with annual precipitation of 800 mm. Mean daily maximum temperature is recorded in July (25.4 ºC) and minimum temperatures in January (-18.4 ºC). The site supported a mature mixedwood stand as described by Brand and Janas (1988) before being clearcut during the summer of 1985.
The experimental design was a randomized 23 factorial complete block design. The factors were a surface soil modification treatment [scarification (S); levels 0 and 1], a fertilization treatment [fertilization (F); levels 0 and 1] and finally, a vegetation control treatment [herbicide application (H); levels 0 and 1]. Level 0 in all treatments represented an undisturbed post harvest condition. Scarification at level 1 (S1) represented removal of logging debris and the entire forest floor organic horizon. Fertilization at level 1 (F1) represented an annual application of Osmocote™ which is a slow-release temperature-dependant fertilizer (17:16:10 NPK plus micronutrients, 9.1% NH4 +-N and 7.9% NO3 --N). Fertilizer was applied each year for six years; 30 g of commercial fertilizer were spread around each tree in the first growing season, gradually increasing to 200 g per tree in the sixth growing season. Herbicide application at level 1 (H1) represented the annual removal of competing vegetation with the herbicide Vision™ (n-phosphonomethyl glycine) applied at 2.0 kg ha-1 of active ingredient in midsummer each year for four years. There were four replicates of each treatment combination for a total of 32 experimental plots. Half of each 20 x 40 m plot was planted with white pine ( Pinus strobus L.) and half with white spruce ( Picea glauca [Moench] Voss); 100 3-yr-old bare-root seedlings per species were planted at 2 m x 2 m spacing in April 1986.
On May 27 1996, a 30 m transect was randomly located in each of the 32 experimental plots, parallel to the longest side of the plot. The transect extended into areas of both tree species. Five sampling locations were randomly distributed along each transect. Organic carbon and total nitrogen were determined from samples taken at the beginning of the growing season, on May 27. Rates of net N mineralization were determined during three different incubation periods. The first two incubations were for six weeks (May 27 to July 7 and July 7 to Augus 18t) whereas the third incubation was for ten weeks (August 18 to October 29). MBC, MBN and soil moisture were determined four times during the 1996 growing season (May 27, July 7, August 18, and October 29)
Soils cores (height = 10 cm, diameter = 7 cm) were collected on May 27th, 1996, at each sampling location (five per transect). The organic layer and mineral soil (0-10 cm) were separated before being transferred into polyethylene bags. Forest floor samples consisted of all organic matter above the mineral soil surface. There was insufficient organic layer in scarified plots to collect and analyze. Composite forest floor and mineral soil samples were air-dried and sieved to 2 mm. Water content of the air-dried samples was determined by further drying the samples (105°C, 48h) before weighing. Quantification of organic matter of forest floor was determined gravimetrically by loss on ignition (Gallardo et al. 1987) whereas organic carbon (Corg) of mineral soil samples was determined by a modified Mobius procedure (Yeomans and Bremmer 1988). Organic matter was converted to Corg by a conversion factor of 0.56 (Nelson and Sommers 1982). Total nitrogen (Nt) of forest floor and mineral soil samples was determined using a Tecator 1030 Macro-Kjeldhal Analyzer (Hoganas, Sweden).
Net N mineralization, ammonification, and nitrification were assessed in the field using an in situ buried bag technique (Eno 1960, Zou et al. 1992, Holmes and Zak 1994). At each sampling point, two soil cores were collected by hammering sharpened PVC pipes (20-cm length, 7-cm diameter) into the soil. The organic layer and surface mineral soil of the first soil core were carefully divided and transferred from the tube into two different polyethylene bags. The bags were then sealed, buried in the hole they were taken from and covered with a 2 cm deep litter (Zou et al. 1992, Munson et al. 1993). The two horizons of the second soil core were also separated and sealed in polyethylene bags to determine initial levels of inorganic nitrogen (NH4 + plus NO3 -). Inorganic N of organic layer and mineral soil (0-10 cm) was extracted in the field according to Van Miegroet (1995). The filtrates were frozen for storage until analyzed; NH4 +-N and NO3 --N were quantified by a Lachat Automated Ion Analyzer. (Lachat QuickChem® 8000, Zellweger Analytics, Inc.).
Organic layer and mineral soil subsamples (sampled from the soil cores) were processed in the laboratory within 48 h after being collected to determine soil moisture, MBC and MBN. Soil MBC and MBN were estimated using the fumigation-extraction procedure (Brookes et al. 1985). Analyzes of extractable Corg were made by dichromate oxydation, using a compact automatic carbon titrator (Mettler DL20) and analyzes of soluble organic N were made using the Tecator 1030 Macro-Kjeldhal Analyzer. A kEC - factor of 0.38 (Vance et al. 1987) was used for converting extractable -C flush to soil MBC, and a kEN -factor of 0.45 (Brookes et al. 1985, Jenkinson 1988) was used for the conversion of extractable –N to MBN. MBN/MBC ratio was estimated based on these data, and the ratio of soil MBC to Corg and soil MBN to Nt were estimated using data from basic soil analyses. All the data are expressed on an oven-dry basis.
Homogeneity of variances and normality of distributions of all data were verified and data that were not homogeneous were natural log transformed prior to analysis. Plot means and plot variances of soil MBC, soil MBN and net N mineralization were analyzed with a two-factor ANOVA with repeated measurements in a complete randomized design. The error term consisted of all block interaction terms, assuming no interaction between blocks and other factors. Seasonal changes in MBC and MBN were analyzed by linear, quadratic and cubic regressions whereas changes in net N mineralization were analyzed by linear and quadratic regressions. The level effect of significance was adjusted for multiple comparisons using the Turkey-Kramer adjustment. Pearson linear correlations were based on plot means, and were performed using data from all treatments and replicates. All statistical procedures were performed using SAS version 6.12 (SAS 1989).
In the organic layer and the mineral soil, the variability of MBC and MBN was important (Table 3.1). Variability of MBC was of the same order as that observed for MBN and variability observed in the organic layer was similar to that of the mineral soil (CV around 30%). In the organic layer, only time influenced MBC variance (Table 3.2), which was significantly lower in August than in other months of the growing season.
In the mineral soil, neither silvicultural treatment nor time affected MBC variance, although it even if unscarified plots tended (p = 0.07) to have lower variance than scarified plots. In the organic layer, neither silvicultural treatment nor time affected variance of MBN. In contrast, both silvicultural treatment and time influenced MBN in the mineral soil (Table 3.2). Fertilization and vegetation control interacted to affect MBN variance. In the mineral soil of fertilized plots, variance of MBN of plots where vegetation control was not carried out was greater than that of plots where vegetation control was applied. On the other hand, in plots with no vegetation control, variance of MBN in fertilized plots was greater than variance in unfertilized plots. For all plots combined, variance of MBN measured in the mineral soil was lowest in August, compared to other dates.
In the organic layer, seasonal variations were evident for both indicators (Table 3.3). MBC increased from 1694 mg C kg-1 soil in May to 3524 mg C kg-1 soil in July, then decreased to 2957 mg C kg-1 soil in October. MBC represented between 0.8 (May) and 1.7 % in July of Corg., MBN also varied over the growing season, but to a lesser extent than MBC (Table 3.1); it increased from 462 mg N kg-1 soil in May to 492 mg N kg-1 soil in July, but then decreased continuously to 419 mg N kg-1 in August, and 311 mg N kg-1 soil in October. There was 1.5 fold more nitrogen immobilized in the microbial biomass in August than in October (P = 0.063). MBN as a percentage of Nt ranged from 3.5 to 5.1; the ratio between maximum and minimum value was 1.4.
Fertilization and herbicide application interacted to affect organic layer concentrations of MBN (Table 3.3); there was more N immobilized in the microbial biomass of plots without herbicide application compared to plots where vegetation control by herbicide application was carried out (Figure 3.1). This was particularly evident for plots where neither vegetation control nor fertilization was applied (Control; harvest only). In these latter plots (F0V0), nitrogen immobilized in the microbial biomass was nearly two- fold higher (p = 0.016) than that of plots where only herbicide application was carried out. MBN was correlated with soil moisture throughout the growing season whereas MBC was only correlated with soil moisture in August. From July to October, soil MBC was positively correlated with soil MBN in the organic layer (Table 3.4) but never in the mineral soil (data not shown).
Figure 3.1 Impacts of fertilization and herbicide application on soil microbial biomass N of the organic layer. (F = fertilization, V = herbicide application; 0 = no treatment applied, 1 = treatment applied)
In the mineral soil neither time nor silvicultural treatment significantly affected MBC (Table 3.5), and the average concentration of MBC was 328 mg g-1. However, seasonal variations were observed for MBN in the mineral soil; MBN gradually increased from May to August (concentrations doubled) and then decreased by the end of October. MBN concentration was lowest in May and highest in August and represented 3.6 % of Nt.
MBN was also affected by the combination of scarification and herbicide application (Table 3.4); without scarification, vegetation control markedly decreased MBN from 34 to 24 mg kg-1, but when scarification was applied, the herbicide treatment had no effect on MBN, compared to the control plot.
The net N mineralization variability was very important, more important than that of microbial biomass (Table 3.1) . Variability observed in the mineral soil was greater than that of the organic layer. In the organic layer and in the mineral soil neither silvicultural treatments nor time affected the variance of net N mineralization rates.
In the mineral soil, immobilization was generally observed, whereas in the organic layer, net mineralization was observed (Table 3.1). The organic layer and mineral soil both exhibited temporal variability in net N mineralization rates but the seasonal patterns differed for the two horizons. Throughout the growing season, net N mineralization rates were approximately 35 fold greater in the organic layer compared to mineral soil. In both soil horizons, time interacted with silvicultural treatments to affect net N mineralization rate (Table 3.3;Table 3.5).
In the organic layer and in the mineral soil, net N mineralization rates were greatest in mid-season (July-August; (Figure 3.2); in the organic layer of unfertilized plots, rates were similar at the beginning and end of the season, while on fertilized plots rates were lower at the end of the season versus the beginning (Figure 3.2a). Fertilization and herbicide application also interacted to affect net N mineralization in the organic layer (Table 3.3). Net N mineralization rates were higher in plots where herbicide was applied (with or without fertilization) and greatest in plots where herbicide was applied as a single treatment (Figure 3.2b). Net N mineralization rates were negatively correlated (r = -0.56, P = 0.054) with soil moisture measured at the beginning of the in situ incubation period.
In the mineral soil, time interacted with both the combination of scarification and fertilization and the combination of scarification and vegetation control (Table 3.5).
Table 3.1 Descriptive statistics (mean and coefficient of variation) for soil microbial biomass C, soil microbial biomass N, and Net N mineralization of the organic layer and the mineral soil (S = scarification, F = fertilization, V = vegetation control by herbicide pulverization; 0 = no treatment, 1 = treatment applied)
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Table 3.2 ANOVA table for variance of soil microbial biomass C and microbial biomass N. Inputs were variances calculated A) by plot or B) by date (S = scarification, F = fertilization, V = vegetation control by herbicide pulverization)
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Table 3.3 Three-way ANOVA table for organic layer microbial biomass C (MBC), microbial biomass N (MBN) and Net N mineralizationrate (NetNmin) (F = fertilization, V = vegetation control by herbicide pulverization; 0 = no treatment, 1 = treatment applied)
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Figure 3.2 Impacts of time and fertilization (a) and herbicide application combined with fertilization (b) on seasonal patterns of Net N mineralization of the organic layer. (H = herbicide application; 0 = no treatment, 1 = treatment applied)
Without scarification, net N mineralization rates of the fertilized plots were significantly greater than that of the unfertilized plots at the end of the growing season (Figure 3.3a), whereas with scarification, the same phenomenon was observed, but at the beginning of the growing season Figure 3.3b). On unscarified plots, net N mineralization rates varied little throughout the growing season whether competitive vegetation was eliminated or not (Figure 3.3a). In the mid growing season net N mineralization rates of plots where herbicide application was combined with scarification were markedly greater than that of plots where scarification was carried out alone (Figure 3.3b).
Table 3.4 Impacts of date on correlations (Pearson’s coefficient) between soil microbial biomass C and N (MBC and MBN respectively) and soil moisture of the organic layer
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Nitrogen immobilization was observed in plots that were neither fertilized or scarified plots at the beginning and end of the growing season (Figure 3.3a), and throughout the growing season on scarified, unfertilized plots (Figure 3.3b). Nitrogen immobilization was only observed at the beginning of the growing season on unscarified, fertilized plots (Figure 3.3a) and at the end of the growing season on scarified, fertilized plots (Figure 3.3b).
Although seasonal patterns of net N mineralization in the organic layer and the mineral soil differed, both patterns closely paralleled the seasonal pattern of net N ammonification rate (r2 = 0.90 p < 0.0001 and r2 = 0.86 p < 0.0001, for the organic layer and the mineral soil respectively).
For this northern mixedwood ecosystem converted to plantation, estimates of MBC and MBN as well as proportion of total C that was immobilized in the microbial biomass (0.8 to 1.7) fell within the range of values observed in studies of temperate forest ecosystems (0.7 to 1.5 ; Carter et al. 1998, Bauhus and Khanna 1999). However, the proportion of total N immobilized in the biomass (3.5 to 5.1) was greater than that generally observed (2.0 – 3.5), particularly at the beginning of the growing season (Christ et al. 1997, Bauhus and Khanna 1999), suggesting that significant proportions of soil N were stored in the microbial biomass.
Figure 3.3 Impacts of herbicide application and fertilization on the seasonal pattern of net N mineralization of the mineral soil of unscarified plots (a) and scarified plots (b). (F = fertilization, H = herbicide application; 0 = no treatment applied, 1 = treatment applied)
Table 3.5 Impacts of single and factorial treatments on microbial biomass C, microbial biomass N (MBC and MBN respectively) and net N mineralization (N-NO3 plus N-NH4) of the mineral horizon (0-10 cm) measured at different times during the growing season. F = fertilization. V = vegetation control by herbicide application, S = scarification, Tl = linear time effect, Tq = quadratic time effect. Values in parentheses are mean standard errors
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Variability of MBC and MBN at any single point in time was important. However, a significant part of this variability may be explained by the method used to quantify these properties. Joergensen et al. (1998) showed that the coefficient of variation for C and N microbial biomass measurements could be important (until 35%), even on replicate measurements. They concluded that the major source of variation was not extraction and fumigation, but the difficulty in measuring low concentrations of C and N in the extracts. This high variability could mask certain effects of time or silvicultural treatments, and only very significant effects may therefore be detected.
MBC and MBN levels in the mineral soil were roughly 90% and 95% (for C and N respectively) lower than those observed in the organic layer. The smallest pools measured in the organic layer were still about four to eight times greater than the largest pools in the mineral soil (for MBC and MBN respectively). Gallardo and Schlesinger (1994) found that seasonal fluctuations of MBC were more pronounced in the organic layer than in the mineral soil. We observed the same phenomenon in our study and this was also true for MBN concentrations. However, time of year explain a maximum of 20% of the variability in MBC values, compared to 8% for MBN in the organic layer and 5% for MBN in the mineral soil (Table 3.3;Table 3.5). Diaz-Ravina et al. (1995) found the same tendency; in their study, soil texture largely explained (more than 80 %) whereas time explain less than 20% of MBC variation. So, if MBC and MBN are used to follow the impacts of perturbations on forest soil over time, it would be more useful to focus on the organic layer and decrease the costs of monitoring. In a qualitative synthesis of temporal microbial biomass data, Wardle (1992) showed that seasonal trends identified in several studies were often contradictory. Different studies reported either positive or negative responses to temporal patterns of soil microclimate (temperature and moisture) and plant productivity. Nevertheless, it seems that sites at higher latitudes are exposed to higher interseasonal variations in microclimate conditions, generally causing greater interseasonal flux of the microbial biomass. The results of the present study, carried out near the latitude 46ºN, confirm this observation.
The temporal pattern of MBC observed in the organic layer was complex, with changes in trends between each sampling date. The seasonal pattern of MBN was more straightforward, decreasing continuously throughout the growing season. The seasonal patterns of soil MBC and MBN may reflect the relative timing and sequence of forest floor physiological events, such as increases in C input from rhizosphere products to the soil (Xu and Juma 1993, Franzluebbers et al. 1994).
As observed by Wardle (1992) and by Holmes and Zak (1994) silvicultural treatments did not longer affect MBC concentrations in the organic layer, indicating that ten years after treatments, original disturbances no longer have destabilizing effects on MBC concentrations. Three and four years after plantation establishment, Munson et al. (1993) and Ohtonen et al. (1992) showed that herbicide application had markedly decreased MBC concentration. This effect on MBC concentration was no longer evident in 1996, indicating a recovery of MBC relative to the control treatment, even if total MBC contents (t ha-1) were still affected by herbicide treatment and fertilization (Périé and Munson, 2000). In contrast, silvicultural treatments and especially the herbicide application continued to affect MBN values.
The N dynamics of terrestrial ecosystems are fundamentally controlled through the population dynamics of soil microorganisms. As such, microbial biomass represents a significant source or sink of available N, the amounts being influenced by the flux of labile C from the above- and belowground production of plant litter (Thomas and Prescott 2000). Therefore, microbial biomass is likely to vary with seasonal changes in factors controlling microbial biomass growth (e.g. temperature, moisture content, substrate availability especially C, and pH) and this should, in turn, influence net N mineralization rates. We found that microbial biomass increased with soil moisture and this can also explain the seasonal trends of the microbial biomass.
Estimates of net N mineralization fell within the range of values summarized for other mixewood ecosystems (Boone 1992) and were in the same order as those observed ten years earlier by Munson et al. (1993), in the same experimental design. However, the nitrate-dominated cycle observed during the first five years of plantation development (Ohtonen et al. 1992, Munson et al. 1993), typical of harvest impacts in certain forest types (Vitousek et al. 1982, Pietikaïnen and Fritze 1995), has been replaced by an ammonium-dominated N cycle (Périé and Munson 1999), which is representative of more mature forest ecosystems (Kimmins 1987, Zak et al. 1989). Net N mineralization displayed a marked seasonal variability, ranging from 222 to 458 mg kg-1 yr-1 during the growing season. As shown elsewhere (Gallardo and Schlesinger 1994, Bauhus and Barthel 1995), seasonal variations were more pronounced in the organic than in the mineral soil; the latter is generally less affected by climatic fluctuations and also contains markedly lower microbial biomass pools. Time alone or combined with the different treatments explained 34 % of the total variation of net N mineralization rate values. Neither MBC nor MBN (measured at the beginning or at the end of the incubation period) was significantly correlated with mean annual rates of net N mineralization. As such, our data do not support the hypothesis that N availability in soil is controlled by large seasonal fluctuations in soil microbial biomass.
There was no correlation between net N mineralization and soil water content (except in Aug-Oct period). Several other studies found that temperature and moisture were the primary controls on the seasonal course of net N mineralization in situ (Vitousek and Matson 1985, Boone 1992). The lack of correlation between net N mineralization and soil moisture may be due to the high degree of within-plot variability observed for the net N mineralization.
In the organic layer, both microbial biomass and net N mineralization exhibited seasonal trends but in the mineral soil only net N mineralization presented marked seasonal fluctuations. In the organic horizon, both MBN and MBC were higher in July but MBC was lowest in May, whereas MBN was lowest in October. Throughout the growing season, MBN was correlated with organic layer moisture but not with net N mineralization rate. Although the seasonal patterns of net N mineralization in the organic layer and the mineral soil differed, both were closely similar to the seasonal pattern of net N ammonification rate. To optimise the monitoring of the impacts of perturbations caused by silvicultural treatments over time on soil quality indicators as microbial biomass and net N mineralization we recommend that:
Measurements of these three indicators should be made only in the organic layer, since activity was markedly greater in the organic layer compared to the mineral soil. This would allow to decrease the cost of monitoring, while focusing on the horizon that is generally most affected by management. Effort could be made to increase the intensity of sampling in the organic horizon.
These three indicators should be measured consistently at the same time in the growing season, since they are characterized by important seasonal patterns and silvicultural treatment impacts can interact with time.
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| 2 TEN-YEAR RESPONSES OF SOIL QUALITY AND CONIFER GROWTH TO SILVICULTURAL TREATMENTS - IMPACTS À MOYEN TERME (10 ANS) DE TRAITEMENTS SYLVICOLES EXTRÊMES SUR LA QUALITÉ DES SOLS FORESTIERS ET LA CROISSANCE DES PLANTS DE CONIFÈRES |
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4 IMPACTS OF HERBICIDE APPLICATIONS ON RELATIONSHIPS BETWEEN MICROBIAL BIOMASS AND ORGANIC LAYER MICROCLIMATE PROPERTIES IN CONIFEROUS STANDS - IMPACTS DE LA PULVÉRISATION D’HERBICIDE SUR LA BIOMASSE MICROBIENNE ET LE MICROCLIMAT DE PEUPLEMENTS RÉSINEUX |