Carbon losses and primary productivity decline in savannah soils under cotton-cereal rotations in semiarid Togo

Abstract

Soil degradation in the savannah-derived agroecosystems of West Africa is often associated with rapid depletion of organic carbon stocks in soils of coarse texture. Field experiments were conducted over a period of more than 30 years at two sites in semiarid Togo to test the impact of agricultural management practices on soil C stocks and crop productivity. The resulting datasets were analysed using dynamic simulation models of varying complexity, to study the impact of crop rotation, fertiliser use and crop residue management on soil C dynamics. The models were then used to calculate the size of the annual C inputs necessary to restore C stocks to thresholds that would allow positive crop responses to fertilisers under continuous cultivation. Yields of all crops declined over the 30 years irrespective of crop rotation, fertiliser use or crop residue management. Both seed-cotton and cereal grain yields with fertiliser fluctuated around 1 t/ha after 20 years. Rotations that included early maturing sorghum varieties provided larger C inputs to the soil through residue biomass; around 2.5 t C/ha/year. Soil C stocks, originally of 15 t ha?1 after woodland clearance, decreased by around 3 t/ha at both sites and for virtually all treatments, reaching lower equilibrium levels after 5-10 years of cultivation. Soil C dynamics were well described with a two-pool SOM model running on an annual time step, with parameter values of 0.25 for the fraction of resistant plant material (K1), 0.15-0.20 for the decomposition rate of labile soil C (K2) and 8-10 t C/ha for the fraction of stable C in the soil. Simulated addition of organic matter to the soil 30 years after woodland clearance indicated that additions of 3 t C/ha/year for 15-20 years would be necessary to build 'threshold' soil C stocks of around 13 t ha?1, compatible with positive crop response to fertiliser. The simulated soil C increases of 0.5 to 1.6% per year are comparable with results from long-term experiments in the region. However, the amounts of organic matter necessary to build these soil C stocks are not readily available to resourcepoor farmers. These experimental results question the assumption that crop residue removal and lack of fertiliser input are responsible for soil C decline in these soils. Even when residues were incorporated and fertilisers used at high rates, crop C inputs were insufficient to compensate for C losses from these sandy soils under continuous cultivation. (Résumé d'auteur

The Effect of Water and Soil Conservation (WSC) on the Soil Chemical, Biological, and Physical Quality of a Plinthosol in Niger

“…Organic resources, however, are limited in the region and not readily available to farmers . Tittonell et al (2012) moreover point out that rehabilitation of degraded soils requires sustained efforts for long periods of time, which discourages smallholder farmers, whereas Kintché et al (2010) and Rusinamhodzi et al (2013) even question whether crop residue management and fertilizer inputs can reverse SOC losses from degraded sandy-soils under continuous cultivation, even when applied at high rates. Lahmar et al (2012) therefore Table II.…”

“…Without adequate inputs, organic matter content has been declining incessantly in various agro ecosystems throughout the world (Hueso‐González et al . ; Laudicina et al ., ; Srivastava et al ., ) and dryland Africa in specific (Kintché et al ., ; Lahmar et al ., ; Mekonnen et al ., ; Mekonnen et al ., ). As a result, the low soil fertility not only impedes sustainable biomass productivity but also renders soils vulnerable to wind and water erosion, prompting further loss of fertile topsoil and organic matter (Cerdà et al ., ; García‐Orenes et al ., ; Mandal & Sharda, ; Zhao et al ., ).…”

“…Resource-constrained farmers often cannot afford external inputs to maintain the soil productivity of continuously cultivated fields, whereas the limited amounts of local crop residues and animal manure have several alternative uses . Without adequate inputs, organic matter content has been declining incessantly in various agro ecosystems throughout the world (Hueso-González et al 2014;Laudicina et al, 2014;Srivastava et al, 2014) and dryland Africa in specific (Kintché et al, 2010;Lahmar et al, 2012;Mekonnen et al, 2015b;Mekonnen et al, 2015a). As a result, the low soil fertility not only impedes sustainable biomass productivity but also renders soils vulnerable to wind and water erosion, prompting further loss of fertile topsoil and organic matter (Cerdà et al, 2007;García-Orenes et al, 2012;Mandal & Sharda, 2013;Zhao et al, 2013).…”

Modelling the continuous exchange of carbon between living organisms, the soil and the atmosphere

“…Many recent studies have modelled the changes in C stocks caused by changes in land management and use over mid-to long-term periods (ranging from a few years to a few decades) where C data are available (Battle-Aguilar et al 2011), such as 10 years (Jenkinson et al 1999) and 30 years (Kintché et al 2010) in a crop rotation system, 5 years (Romanyaa et al 2000) and 18 years (Palosuo et al 2012) in forest systems (Farage et al 2007;Nieto et al 12 years after fire and drought in grasslands and shrublands (Martí-Roura et al 2011), 13 years under tillage and N fertilization changes (Alvaro-Fuentes et al 2012) and 24 years of organic amendments (Heitkamp et al 2012). …”

Projection of Soil Organic Carbon Reserves in the Argentine Rolling Pampa Under Different Agronomic Scenarios. Relationship of these Reserves with Some Soil Properties

“…The AMG model is a simple three-compartment model. This model has been calibrated and validated in the Rolling Pampa in two long-term experiments under conventional tillage and NT by applying the natural 13 C abundance technique [1] and in different long-term tillage systems and crop sequences experiments under different soil and climate conditions [22,[36][37][38][39]. Lal [19] reported that although significant progress has been made in predicting the SOC compartment under different changes in land use and management, efforts should be made to include in such models predictions concerning changes in soil quality regarding variations in the SOC compartments and flows.…”