Soil organic matter decomposition response to N enrichment in grassland soils
Soils are a large reservoir of organic, fixed C; however, the effects of nutrient enrichment on this significant C stock are poorly understood. Previously, we found that nitrogen (N) addition increased the rate of decomposition of the “fast” cycling soil C pool and decreased the rate of decomposition of the “slow” cycling soil C pool in grassland soils. Although these results parallel those from leaf litter decomposition studies, the biological and chemical mechanisms that underlie the response of soil organic matter decomposition to added nutrients remain uncertain. To address this gap, we quantified the effects of nutrient enrichment on microbial respiration, microbial enzyme activity, and microbial growth efficiency at three Nutrient Network sites in the US Central Great Plains that have received multi-factorial additions of N, phosphorus (P), and/or potassium plus micronutrients (K+) since 2008. We hypothesized that: 1) N addition increases decomposition of the “fast” soil C pool by increasing the activity of hydrolytic enzymes; and 2) N addition decreases decomposition of the “slow” soil C pool by decreasing the activity of oxidative enzymes and increasing microbial growth efficiency.
In contrast to our previous research and our first hypothesis, we found that N addition had neutral or negative effects on the decay rate of the “fast” pool (site x N interaction, p < 0.0001). Furthermore, hydrolytic enzyme activity did not consistently increase with N addition (significant site x N x P x K+ interaction for all enzymes). In line with our previous research, we found that N addition decreased the decomposition rate of the “slow” soil C pool (p < 0.0001). However, our oxidative enzyme activity data did not support our second hypothesis: we found there was a positive effect of N addition on the activity of one oxidative enzyme (peroxidase) when added with P (N x P interaction, p = 0.03) and no effect of N addition on the other oxidative enzyme (phenol oxidase, p > .1). These results contrast with studies of forests soils that report decreased soil organic matter decomposition in response to N due to reductions in oxidative enzyme activity. Instead, our results suggest that in grassland soils, N effects on enzyme activity cannot explain decreased soil organic matter decomposition with N addition.