Do long-term N additions affect the soil organic carbon pool in temperate grasslands?

While the organic carbon stored in soil is a sizeable proportion of the total carbon stored in terrestrial ecosystems, it is also a considerable source of greenhouse gas emissions. In this study, we examined how the carbon pool had changed in an area of temperate grassland in Inner Mongolia, China, over a period of 14 years. A field experiment was set up in 2003 and was treated with 6 nitrogen (N) treatments:0, 2, 4, 8, 16, and 32 g·N·m^-2·yr^-1 as dry urea (CO(NH₂)₂). We collected soil samples in 2017, and divided them into three size aggregates: silt-clay fraction, microaggregate, and small macroaggregate. We determined various soil extracellular enzyme activities of these three categories, namely β-glucosidase (BG), N-acetyl-β-D-glucosaminidase (NAG), acid phosphatase (AP), peroxidase (PER), and phenol oxidase (POX). We found that the soil enzyme activities increased under N additions. In addition, the BG activity was higher, but the PER activity was lower, in the small macroaggregates than the silt-clay fraction. Furthermore, we found that the soil extracellular enzyme activities and soil physico-chemical properties in the small macroaggregates were not correlated. Rather, we found that some of the soil extracellular enzyme activities were negatively correlated with the pH value, microbial biomass carbon (MBC), total organic carbon (TOC), and positively correlated with the inorganic nitrogen content (IN), in the microaggregates and the silt-clay fraction. A key discovery was that the N additions had no effect on the 0-10 cm soil layer (fractions <2000 um) organic carbon pool or the distribution of different-sized aggregates, probably because they were regulated through the changes of plant, soil, and microbial interactions. The results will contribute to improve our understanding of how N additions affect TOC and different aggregate size classes in soil, and will support better predictions of how N deposition might contribute to future climate change.