Soil Quality Indicators, Building Soil Organic Matter and Microbial Derived Inputs to Soil Organic Matter under Conservation Agriculture Ecosystem: A Review

Agricultural intensification is placing tremendous pressure on the soil’s capacity to maintain its functions leading to large-scale ecosystem degradation and loss of productivity in the long term. Therefore, there is an urgent need to find early indicators of soil health degradation in response to agricultural management. Our purpose was to review the literature in which a wide perspective of soil quality and the complex task of its assessment, considering the inherent and dynamic factors, are introduced. It focuses on the possibilities of applying and integrating the accumulated knowledge in agro-ecological land evaluation in order to predict soil quality. Land-use change, especially from conservation agriculture ecosystem (CA) to intensive agriculture, is negatively impacting soil quality and sustainability. Soil biological activities are sensitive indicators of such land-use impacts. Land use and management practices affect microbial properties in topsoil but have no effects in subsoil. Total organic C and N contents as well as microbial biomass were significantly higher in CA compared with conventional farming. The tillage treatments significantly influenced soil aggregate stability and OC distribution. Soil OC and MBC were at their highest levels for 1.0–2.0 mm aggregates, suggesting a higher biological activity at this aggregate size for the ecosystem. Compared with CT treatments, NT treatments increased MBC by 11.2%, 11.5%, and 20%, and dissolved organic carbon (DOC) concentration by 15.5% 29.5%, and 14.1% of bulk soil, >0.25 mm aggregate, and <0.25 mm aggregate in the 0−5 cm soil layer, respectively. Increased SOC stock in the surface 50 kg m-2 under ZT and PRB was compensated by greater SOC stocks in the 50-200 and 200-400 kg m-2 interval under residue retained, but SOC stocks under CT were consistently lower in the surface 400 kg m-2.Soil organic carbon fractions (SOC), microbial biomasses and enzyme activities in the macro-aggregates are more sensitive to conservation tillage (CT) than in the micro-aggregates. Crop residues and rhizodeposits support higher microbial biomass, leading to enhanced enzyme activities in conservation agriculture ecosystem soil. Responses of macro-aggregates to straw return showed positively linear with increasing SOC concentration. Straw-C input rate and clay content significantly affected the response of SOC. Overall, straw return was an effective means to improve SOC accumulation, and soil quality. Straw return-induced improvement of soil nutrient availability may favour crop growth, which can in turn increase ecosystem C input. Tillage reduction and residue retention both increased the proportion of organic C and total N present in soil organic matter as microbial biomass. Microbial immobilization of available-N during the early phase of crops and its pulsed release later during the period of greater N demand of crops enhanced the degree of synchronization between crop demand and N supply. Overall, it indicates that land-use change significantly alters microbial properties in topsoil, with modest effects in subsoil. Microbial properties should be considered in environmental risk assessments as indicators of ecosystem disturbance caused by land-use and management practices.