Effects of nutrient enrichment on size and cycling of multiple soil organic matter pools

Background/Question/Methods

Humans have increased the creation and deposition of biologically reactive nutrients worldwide. However, we know little about how nutrient enrichment will influence rates of soil organic matter decomposition in traditional “fast” and “slow” cycling pools, together a significant global reservoir of carbon. Our research aims to examine the influence of nutrient enrichment on microbial decomposition of organic matter, as well as on mechanisms that inhibit microbial access to organic matter (e.g. through occlusion in soil aggregates and adsorption to mineral surfaces). We hypothesize that nutrient addition will: 1) increase the decomposition of un-occluded organic matter through decreased microbial nutrient limitation, 2) decrease aggregate-occluded organic matter through decreased root and microbial biomass, which contribute to aggregate formation, and 3) increase mineral surface-occluded organic matter through changes to mineral surface reactivity. To address these hypotheses, we sampled five participatory grassland sites of the Nutrient Network that have received multi-factorial additions of nitrogen (N), phosphorous (P), and potassium (K) since 2008. Sites span a soil texture and climatic gradient across the U.S. Central Great Plains region. Soils were subjected to long-term soil respiration incubations and soil fractionation to examine nutrient addition effects on both fast and slow cycling soil organic matter.

Results/Conclusions

The decomposition rate of un-occluded soil organic matter across a five month incubation increased with N (p < 0.0001) and P (p = 0.057) addition. Despite significant declines in microbial biomass with N addition (p = 0.0001), initial analysis of aggregate bound organic matter found no significant effect of N on soil macro- and micro-aggregate carbon content. Similarly, we saw no effect of N addition on mineral-occluded carbon content although nutrient addition decreased soil pH (p = 0.036), which likely influences mineral surface reactivity. Since un-occluded and occluded organic matter have distinct mean residence times in soil, the contrasting effects of nutrient enrichment on these pools suggest that the short- and long-term effects of nutrient addition will differ.