Effects of tillage on soil N availability, aggregate size, and microbial biomass in a subtropical karst region

Tillage results in the loss of soil nitrogen (N) over the short term in a typical karst area, but the factors affecting soil N content under intensive agriculture are poorly understood. In this study, our objective was to examine the effect of tillage at various frequencies on soil aggregates and microbial biomass, and consequently on soil N content. A one-year in situ simulation experiment was conducted using five tillage treatments: no tillage (T0), semiannual tillage (T1), and tillage every four months (T2), two months (T3), and one month (T4). Tillage at various frequencies reduced the total N (TN) content of soil from 0.43 to 0.72 g kg(-1), compared with the no-till treatment. Nitrate (NO3-) was the major form of dissolved N in this karst region probably owing to high soil pH and it declined sharply with increasing tillage. Tillage disintegrated large soil aggregates (5-8 mm). Microbial biomass carbon (MBC) decreased gradually with increasing tillage frequency, while microbial biomass nitrogen (MBN) was sharply reduced by all tillage treatments including T1. In addition, redundancy analysis (RDA) indicated that both 5-8 mm-sized aggregates and MBC significantly impacted the N cycle (P < 0.05). Structural equation model (SEM) analysis showed that tillage significantly affected total N loss (Delta TN) via the fragmentation of 5-8 mm aggregates. These findings highlight the importance of preserving large aggregates for maintaining the soil N pool in fragile ecosystems, and preventing the loss of soil N due to tillage in the karst region, as its high soil NO3- content and likely leaching may be a threat to groundwater quality.