Long-term conservation tillage and nitrogen fertilization effects on soil aggregate distribution, nutrient stocks and enzymatic activities in bulk soil and occluded microaggregates

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.