Abstract
Chronic synthetic nitrogen (N) application can result in a significant accumulation of nitrate in the subsoil, which could alter subsoil N cycle and subsequently affect subsoil N levels. To understand how elemental interactions affect the cycle and storage of subsoil N, we examined the soils receiving no fertilizer control (CK), 30-year applications of synthetic fertilizer (CF), and CF plus organic manure (CF + OM). The N cycling microbial groups and activity were investigated through analyzing abundance of bacteria, nitrifiers and denitrifiers, potential nitrification (PNA) and denitrification (DEA) rates in the topsoil (0–20 cm) and subsoil depths (20–80 cm). Compared with the CK, the CF application increased subsoil nitrate but reduced or did not change subsoil microbial biomass N and total N. Corresponding to the increased nitrate, the abundances of denitrifiers increased in the CF subsoils. By contrast, the abundances of nitrifiers increased in the CF topsoil. Significant correlation between the abundances of nitrifiers and soil PNA was found in the topsoil, while significant correlation was also found in the subsoil between the abundances of nirS- and/or nirK-type denitrifiers and DEA. These results suggest that the depleted or less changed subsoil total N by CF application might be partly related to the enriched denitrifiers groups and the related potential activity. The contrasting responses of nitrifiers and denitrifiers in the CF subsoil indicate a decoupling of both processes. Our findings highlight that the leached nitrate by synthetic fertilizer addition not only occurs as an environmental risk causing groundwater contamination but may also alter the subsoil N cycle through the denitrifier groups.
Highlights
Nitrate leaching and accumulation in soil profiles have been found in crop lands worldwide (Ju et al, 2004; Stumborg et al, 2007; López-Bellido et al, 2013), which is mainly caused by the chronic application of synthetic nitrogen (N) fertilizer in improving crop yield
The percent change analysis revealed that the rate of change induced by CF and CF + organic farmyard manure (OM) was generally greater for soil available P, nitrate, MBN, potential nitrification (PNA), and DEA than for TOC, dissolved organic C (DOC), MBC, and soil microbial respiration (Figure 2)
Our results showed that N cycling rates (PNA and DEA) in the subsoil were more responsive to fertilizer application than C-cycle-related variables (Figure 2)
Summary
Nitrate leaching and accumulation in soil profiles have been found in crop lands worldwide (Ju et al, 2004; Stumborg et al, 2007; López-Bellido et al, 2013), which is mainly caused by the chronic application of synthetic nitrogen (N) fertilizer in improving crop yield. The increased nitrate in subsoil profile is Denitrifiers in Fertilized Soil Profile likely to affect soil N pools and cycling rates (Treseder, 2008; Lu et al, 2011; Jian et al, 2016), which might alter soil carbon (C) pools and cycling. Such impacts are important for the regulation of soil fertility in the soil profile (Schmidt et al, 2011; Shang et al, 2014; Jian et al, 2016). The depleted or less changed subsoil total N by fertilization may be more tightly coupled to microbial processes than to N input per se
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