Land-use type affects nitrate production and consumption pathways in subtropical acidic soils

The production and consumption pathways of nitrate (NO3--N) are key factors for NO3--N retention capacity (NR) in soils, especially in subtropical regions with high precipitation. This study was designed to examine the effect of land-use type on NO3--N production and consumption pathways, in acidic soils in a subtropical region. Soils were collected from 5 differing land-use types: shrubland (SB), a eucalyptus plantation (ET), a sweet potato farm (SP), a citrus orchard (CO) and a mixed-vegetable farm (VE), in southeast China. In our study, SB was considered a natural system, ET was considered an artificial plantation system, and SP, CO and VE were considered agricultural systems. An incubation experiment was conducted with two N-15 tracing treatments, and a numerical modeling method was applied to the experimental data to quantify gross N transformation rates. The results showed that 76% and 69% of NO3--N was produced by heterotrophic nitrification (O-Nrec) in SB and ET respectively. Whereas, O(N)rec was negligible in SP, CO and VE, with > 99% of NO3--N produced by autotrophic nitrification. Dissimilatory NO3--N reduction to NH4+-N (DNRA) had an important role in soil NO3--N consumption in SB and ET, with > 90% of NO3--N produced in SB being simultaneously consumed via DNRA. However, DNRA was unimportant in SP, CO and VE. Nitrate immobilization and denitrification (I-NO3) was negligible for all land-use types. Soil NR under different land-use types was in the order SB > ET > SP approximate to CO approximate to VE. Although N fertilizer was applied at low rates, for only the following two years after planting eucalyptus saplings, the NR in ET was significantly lower than in SB (natural system). Nitrate retention capacity was negatively correlated with soil organic carbon (SOC) and total nitrogen (TN) concentrations, and positively correlated soil C:N and C: NO3--N ratios. The lower NR in SP, CO and VE may therefore be due to higher SOC and TN concentrations and lower C:N and C:NO3--N ratios in those soils relative to SB and ET soils. Our observations imply that O-Nrec. and DNRA play an important role in soil NO3--N production and consumption in SB and ET land-uses, but not in agricultural land-use (SP, CO and VE), within the subtropical region of China studied. These findings could contribute to a theoretical and practical framework for managing NO3--N in subtropical acidic soils.