Impact of willow-based grassland alley cropping in relation to its plant species diversity on soil ecology of former arable land

Abstract Effects of silvo-arable alley cropping systems (ACS) on soil functions have frequently been investigated, however, less is known about the effects of silvo-grassland ACS. Conversion of arable land to grassland ACS may have a high capability to rebuild soil fertility, while increased grassland plant diversity may further foster improvements. The objective of the study was to assess the impact of willows (Salix spp.) and grassland plant species diversity on soil ecology of an ACS established on former arable land. Thus, soil quality indices, like soil organic carbon (SOC), microbial biomass C and N, fungal abundance and microbial functional diversity, were quantified in formerly ploughed Eutric (Stagnic) Cambisols in two soil depths (0–5, 5–20 cm) of a temperate grassland ACS (Lower Saxony, Germany). To evaluate potential tree effects on alleyways, distance transects were analysed by repeated measures mixed effects models, considering abiotic factor (pH, clay content) variability. Possible changes of soil quality indices within the former ploughed soil layer were deduced by a comparison of both soil depths at each distance. Linear contrasts were calculated for comparisons of grassland diversity levels. Results showed significantly higher SOC contents (16.2–16.9 mg g−1) and soil microbial properties (e.g. MBC: 453-462 μg g−1, ergosterol: 4.3-4.4 μg g−1) in upper topsoils under trees and grassland compared with lower topsoil layers (11.6–12.0 mg g−1, 262-277 μg g−1 and 1.7-1.8 μg g−1 for SOC, MBC and ergosterol, respectively). No effects of plant species diversity on microbial properties have been detected to date. Similar levels of soil quality indices in upper topsoils under trees and grassland are ascribed to the cessation of tillage and a permanent vegetation cover ongoing for 5 years, as aboveground tree litter inputs into the alleyways were minor. Hence, irrespective of the diversity level of grassland vegetation, grassland ACS may have a high potential to increase soil quality for matching the requirement of sustainability of agroecosystems. However, as solely topsoils have been investigated, henceforth, the consideration of subsoils is vital to assess the effect of deep tree roots in grassland ACS.