دانلود رایگان مقاله استوکیومتری منابع بواسطه از دست دادن C خاک

Abstract

Root exudation is increasingly being recognized as an important driver of ecosystem processes; however, few studies have examined the degree to which variations in exudate stoichiometry and soil resources affect microbial controls of nutrient availability and decomposition. We added root exudate mimics of varying chemical quality to soils collected from two adjacent forest stands (one a
70 year-old spruce plantation, the other a
200 year-old spruce-fir forest) that differ strongly in N availability. The exudate treatments were added for 50 consecutive days, and included water (control), C alone, N alone, and three combinations of C and N that varied stoichiometrically (i.e., C:N ratio of 10, 50 and 100). Exudate additions containing little or no N promoted the greatest losses of soil C in two soils, with the greatest losses occurring in the moderately labile (i.e., acid-extractable) fraction of the low N plantation soils. However, despite the uniformity of priming effects between sites (
7% loss of soil C for both), there was little congruence in exudate-induced effects on microbial biomass and activity. In the plantation soils, exudates generally increased microbial biomass (especially fungi), accelerated N cycling and increased lignin-degrading enzyme activities relative to controls. In contrast, exudate additions to spruce-fir soils mostly decreased microbial biomass, decelerated N cycling, and had variable impacts on lignindegrading enzyme activities (decreased phenol oxidase but increased peroxidase). Collectively, this study suggests that while root exudates with low C and N have the potential to accelerate soil C losses by stimulating microbes to mine N from soil organic matter, the consequences of exudate inputs on nutrient fluxes are less predictable, and may hinge on the recalcitrance of (soil organic matter) SOM, N availability and microbial communities.

4. Discussion

Knowledge of how exudate stoichiometry interacts with soil N availability to influences priming effects and nutrient fluxes is critical for predicting ecological consequences of root-microbe interactions on soil C-balance, particularly in the context of global environmental change. Here the results show that root exudate mimics containing C (but little to no N) induced the greatest losses of soil C. However, despite similarities between the two sites in terms of soil C losses, exudate inputs had divergent effects on SOM fractions, microbial processes and nutrient fluxes. Consequently, the results suggest that exudate-induced priming effects on C-nutrient couplings can be driven by multiple factors, such as the recalcitrance of SOM, N availability and soil microbial communities (Fig. 5).