Long-term conservation tillage and nitrogen fertilization effects on soil aggregate distribution, nutrient stocks and enzymatic activities in bulk soil and occluded microaggregates
Author(s)
Date Issued
2020
Type
article
Volume
196
Start Page
104482
Journal
Abstract
Conservation agriculture is globally recommended for increasing soil C and reducing greenhouse gases emissions by modifying soil physical, chemical and biological properties and processes. We assessed the impact of long-term conservation tillage and N fertilization in wheat-soybean rotation on soil aggregate distribution, nutrients stocks and functions of soil microorganisms related to C, N, P and S cycles at different aggregation scales in the Mediterranean area. A long-term field experiment was set up in a silt loam soil comparing two tillage intensities to bread wheat (Triticum aestivum L.) - soybean (Glycine max L. Merr.) rotation: conventional tillage (CT) and minimum tillage (MT). Two N fertilization levels were applied on bread wheat: 0 and 200 kg N ha−1 (N0 and N200, respectively). Under CT, almost 100% of the crop residues were incorporated in the 0–25 cm soil layer, whereas under MT approximately 50% were incorporated at 0–15-cm depth. Tillage was the most discriminant factor explaining 72% and 60% of total variance of soil parameters at 0–15 cm and 15–30 cm soil depth, respectively, whereas N fertilization explained 22% and 29% of total variance, respectively. All enzyme activities were higher under MT, whereas the majority of soil chemical parameters were higher under N200. Under MT a higher proportion of free microaggregates (+51%) was recorded compared to CT suggesting that in a silt loam soil MT had a greater potential to form macroaggregates. The proportion of small macroaggregates was not changed by tillage, but when this fraction was fractionated a higher proportion of occluded microaggregates was found under MT at 15–30 cm (+21%). In the occluded microaggregates, interaction of tillage x N fertilization explained 52% of total variance of soil parameters at 0–15 cm. The most discriminant parameters were the biochemical ones and SOC. All those parameters were higher in MT-N200. Specifically, SOC in occluded microaggregates was increased by N fertilization under MT (+16% at 0–15 cm; +84% at 15–30 cm), whereas it was decreased under CT (-46% at 0–15 cm; -15% at 15–30 cm). In addition, the synthetic indexes for enzymatic activities and for those involved in C-cycle were reduced by N fertilization under MT up to 37% and increased up to 87% under CT. In the Mediterranean area, maintaining or even increasing SOC conservation may require both reduced tillage systems and N fertilization, shifting microbial community towards toward taxa more effective in contrasting soil degradation.