اثر مدیریت خاکورزی و باروری بر ماده آلی خاک و راندمان سورگوم در منطقه نیمه خشک غرب آفریقا

Whether it is traditional, modern or “sustainable” agriculture, soil organic matter plays a key role in sustaining crop production and in preventing land degradation. A field experiment was conducted on a Ferric Lixisol at Gampela (Burkina Faso) in 2000 and 2001 to carried out the effects of tillage, fertilisation and their interaction on soil organic carbon (SOC) (0–10 cm), crop performance and microbial activities. Maize straw or sheep dung were applied separately or combined with urea in a till or no-till systems and compared with urea only and a control treatment. Sampling was done each year at 2 months after sowing and at harvest. SOC was increased in the tillage treatments in 2000 by 35% but only with 18% in 2001 suggesting reduced carbon accumulation in the absence of organic and mineral restitution. Ploughing in maize straw under conditions of N deficiency led to a drastic decrease in SOC due microbial priming effect that, was not observed when ploughing in sheep dung. In no-till system, losses, organic amendment N concentration and the soil N status determined the impact on SOC and crop productivity. The negative effect on SOC in the tillage treatment with maize straw (4.1 g kg−1) was less when maize straw was combined with urea (6.2 g kg−1). It is concluded that in semi-arid West Africa, without both organic resource and N inputs, soil organic matter “pays” for crop N nutrition. Increasing SOC accumulation while improving crop yield may be conflicting under low-input agricultural systems in semi-arid West Africa. Therefore, optimum soil organic carbon and crop performance results from a judicious combination of organic resources and inorganic N mediated by microbial activity.

Soil organic matter (SOM) plays a key role in the improvement of soil physical, chemical and biological properties. Many studies have shown that the addition of organic material improves soil physical properties, which enhance root development, resistance to erosion, soil porosity and water infiltration and decrease soil crusting (Mando and Miedema, 1997 and Fernandes et al., 1997). SOM is a source of nutrients and energy for the decomposer community and a source of nutrients for plant growth (Paul and Clark, 1996). Application of organic resources leads to the improvement of crop yields as a result of improved soil properties (Ouédraogo et al., 2001, Scholes et al., 1997 and Mando, 1998). However, continuous cultivation in conjunction with residue removal, fertiliser application and tillage are often mentioned as causing rapid mineralisation of SOM within the first few years of cultivation (Woomer and Swift, 1994 and Scholes et al., 1997). Doran et al. (1998) showed that no-till management resulted in the lowest loss of SOC and N in the topsoil over time as compared to tilled soils. However, under semi-arid conditions, hard data to clearly establish the role of the quality of organic inputs, tillage and N fertiliser application on SOM accumulation and decomposition are scarce. Combining organic resources and mineral fertiliser has been recommended to increase not only the total above ground biomass but also the below ground biomass production (roots) which can result in an increase of soil organic matter (Bationo and Burkert, 2001). An important issue is whether combining two technologies (here tillage and fertilisation) gives only additive benefits (i.e., the benefit of the combined application is equal to the sum of the benefits from the two technologies when applied in isolation) or truly leads to a positive or negative interaction (Iwuafor et al., 2002). This paper investigates the impact of organic resource quality, fertiliser and tillage and their interactions on SOC and crop performance in semi-arid West Africa during two consecutive year cropping. We hypothesise that combining organic resources with N fertiliser may mitigate SOC loss in cropping systems.

Ploughing in low quality organic resources under conditions of N deficiency leads to a drastic decline in SOC. Without both organic and N inputs, soil organic matter pays for the cost of crop N nutrition. Combining crop residues with N fertiliser yields better results in SOC accumulation and in suppressing the soil microbial depressive impact on SOC. With respect to the hypothesis formulated at the start of the research, increasing SOC accumulation with the use of crop residues in tilled systems in the semi-arid conditions of West Africa is only valid if soil N status is improved. The results show that improving crop yield when enhancing SOC at the same time may be conflicting under low external input agricultural conditions in West Africa. It is concluded that optimum soil organic carbon and crop performance results from a judicious combination of organic resources and inorganic N mediated by microbial activity.