Lessons Learnt from Long-term No-till Systems Regarding Soil Management in Humid Tropical and Subtropical Regions

Abstract

Based on the thermodynamic principle of minimum entropy production, we propose that the two fundamentals of soil management in the sunny, warm and rainy ecosystems of humid tropical and subtropical regions are non-disturbance of soil and high input of crop residues. The positive results of ordering soil processes surpassing dissipative ones are clearly enabled by no-till (NT), but not totally. High input cropping systems, adding at least 10 Mg DM ha−1 year−1 of phytomass to soil, must be properly coupled to NT in those regions as well. Soil organic matter (SOM) is the nexus between a conservation management system, like high input NT, and the resulting improvement of soil quality that leads to sustained crop production and environmental conservation. Particularly in the tropics and subtropics, SOM plays crucial roles in soil aggregation, water holding capacity, CEC, nutrient storage, biological activity, and many other soil processes. In the first 5–10 year of a conservation management system, SOM accumulates mainly in the top 0.2 m soil, but in the long term (>20 years) accumulation also extends to layers as deep as 0.20–1 m, thus prolonging the period of accumulation to more than the 20–30 years that were initially expected for conservation managements in temperate regions. With substantial soil carbon accumulation, and in many cases mitigation of soil nitrous oxide emissions, conservation management has also helped to curb greenhouse gases emissions. Yet NT has a number of critical challenges in tropics and subtropics, many of them associated just with cropping system. Autumn and or winter fallow, for instance, prevent the achievement of the minimum 10 Mg DM ha−1 year−1 of phytomass addition, besides leaving the soil prone to the heavy and erosive rainfalls. Cultivation of cover crops is a feasible strategy to close these fallow gaps, including the cultivation of legumes, which are also advantageous to nitrogen input and even to SOM accumulation. New insights on the increased efficiency of soil microorganisms to metabolize carbon from labile residues such as legumes and on the stabilization of this carbon in organo-mineral interactions are crucially supportive to the recommendation of these species in sustainable NT cropping systems. Other challenges of NT relate to soil compaction and the low adoption of support practices such as contouring and terracing. Our key message is that NT is an outstanding farming system in tropical and subtropical regions that enables the non-disturbance of soil and the many other benefits that follow, but its successful outcomes in terms of organic matter accumulation and therefore minimum entropy production are only attainable in cropping systems with a high input of crop residues; an input that is ecologically achievable, given the favorable conditions and resources of humid tropical and subtropical regions.