Sustainability under combined application of mineral and organic fertilizers in a rainfed soybean–wheat system of the Indian Himalayas

Abstract

Soybean ( Glycine max L.)–wheat ( Triticum aestivum L.) rotation is one of the profitable cropping systems under rainfed conditions in the sub-temperate agro-ecosystem of the Indian Himalayas. We measured the long-term sustainability of the system for farmyard manuring and mineral fertilizer input practices utilizing the trends in grain yield, partial factor productivity (PFP), agronomic efficiency (AE), benefit:cost ratio (B:C ratio), soil organic C (SOC), total N content, available nutrient (P and K) status, microbial biomass C, dehydrogenase activity, selected soil physical properties (bulk density, soil water retention and infiltration rate) and sustainable yield index (SYI). The long-term (30 years) soybean–wheat experiment was conducted at Hawalbagh, Almora, India in a sandy loam soil (Typic haplaquept) under sub-temperate climatic conditions. Every year, the nutrients were applied to the soybean crop and wheat was grown without addition of any external sources of nutrients (residual wheat). The unfertilized plot supported a 0.56 Mg soybean yield ha −1 and a 0.71 Mg wheat yield ha −1 (mean yield of 30 years). Soybean responded to inorganic NPK application and the yield increased to 0.87 Mg ha −1 with NPK. Maximum yields of soybean (2.84 Mg ha −1) and subsequent wheat (1.88 Mg ha −1) were obtained in the plots under NPK + farmyard manure (FYM) treatment. Soybean and residual wheat yields and PFP in the plots under the unfertilized and the inorganic fertilizer treatments decreased with time, whereas they increased in the plots under N + FYM and NPK + FYM treatments for both crops. The data revealed that SYI and AE of fertilizers were also greater in plots receiving fertilizer N or NPK along with 10 Mg ha −1 FYM. Benefit:cost ratio of fertilization increased in both soybean and residual wheat for the plots under N + FYM and NPK + FYM, indicating that the benefit accruing from fertilization improved with time. After 30 years, total SOC and total N concentrations were increased in all treatments. Soils under NPK + FYM-treated plots contained higher total SOC and total N by 104% and 86% in the 0–15 cm soil layer, respectively, over that of initial soil. These differences in the total C content caused profound differences in soil physical properties. Available water capacity and water transmission characteristics in the soil were highest in the NPK + FYM treatment and lowest in the unfertilized treatments, with the mineral fertilizer treatments showing intermediate results. Combined use of NPK and FYM increased SOC, total N, Olsen P and ammonium acetate exchangeable K by 47%, 31%, 13% and 73%, respectively, in the 0–15 cm soil layer, compared to application of NPK through inorganic fertilizers. The application of NPK + FYM also showed the highest levels of soil microbial-biomass C and dehydrogenase activity. As total SOC and total N were increased after 30 years of the experiment in all the treatments, the decline in yields was probably due to a decline in available soil P and/or K. The results clearly reveal that current mineral-fertilizer recommendations are inadequate, whereas annual application of FYM along with NPK fertilizers sustains yield and soil productivity.