Chickpea nitrogen fixation, rhizosphere nutrient concentration and contribution of residual nitrogen to improve maize production in response to biochar application in three different soil types

Abstract

Soil degradation is a major challenge affecting agricultural production around the world in the twenty-first century. Alternative approaches including the use of biochar and the introduction of legumes that will fix nitrogen though the process of biological nitrogen fixation (BNF) are essential for improving soil quality of current low productive soils, thus increasing crop yields, and maintaining food security while conserving the environment. To address this problem, three experiments were conducted in this study. First, two locally produced biochar from poultry litter and acacia feedstocks were assessed whether they qualify as ‘biochar’ for use as soil amendment according to the international biochar bodies. Secondly, a pot experiment was then conducted to investigate the potential impact of poultry litter biochar (PLB) and acacia biochar (ACB) to improve rhizospheric soil nutrient availability, bacteria abundance and diversity, chickpea growth and total nitrogen fixation in three contrasting soil types. Thirdly, another pot experiment was conducted to determine the contribution of residual N from biochar and N-fixed by chickpea to the following maize crop grown in three contrasting soil type. The treatments consisted of three soil types classified as Fernwood [Arenosol]; Pinedene [Gleyic Acrisol]; Griffin [Helvic Acrisol], sampled from three different smallholder farmers at Dopeni village, Limpopo Province. The two biochars [PLB and ACB] were applied at four application rates of [0% (control), 0.5, 1 and 2% w/w]. The treatments were arranged in a completely randomized design and replicated four times. For the first experiment, chickpea (Cicer arietinum) desi cultivar was inoculated and grown for 65 days in soils with uniformly applied P at 60 kg P/kg in all pots and water maintained at 60% field capacity. For the second experiment, maize (Zea mays) was grown for 95 days in the same soil as chickpea and biochar treatments as well as after harvesting maize that was used as a reference crop. Biochars made from poultry litter and acacia feedstocks meet the International Biochar Initiative (IBI) and European Biochar Certificate (EBC) requirements and qualifies as biochar for use as a soil amendment. Both biochars had C content greater than 50%, with H/C and O/C ratios less than 0.6 and 0.4, respectively, indicating that both biochars are stable for C sequestration and can remain in the soil for about 1000 years. Because poultry litter biochar (PLB) contained more nutrients than acacia biochar (ACB), PLB improved rhizospheric pH, CEC, and nutrient concentration (N, P, K, and Ca) when applied at 0.5-2% in the Griffin and Pinedene soils, resulting in higher biomass production and nutrient uptake of chickpea. In addition, when 2% PLB was applied to those two soils, bacteria capable of fixing N, especially those from the phylum Proteobacteria, were more abundant. Thus, chickpea grown in these soils and at these PLB rates derived more N from the atmosphere, fixed more N, and xvi accumulated more N and C in the shoot, but was less water use efficient. Furthermore, maize grown in 1% residual PLB treatments produced more biomass and accumulated more N and other nutrients than maize grown with 0.5-2% residual ACB treatments. However, when grown after chickpea harvest in residual PLB treatments of 2% in the Griffin and Pinedene soils, maize produced greater biomass and accumulated more N and other nutrients. Application of PLB and ACB at 0.5% in the Fernwood soil was ideal to improve rhizospheric nutrient availability, the abundance of bacteria communities [from the phylum Proteobacteria, Acidobacteria, and Firmicutes which are important for C and N cycling and bioremediation], as well as growth, BNF, and C accumulation of chickpea, including maize growth and nutrient uptake in monocropping or in rotation with chickpea. The greatest variation in relative abundance of bacteria communities and growth of chickpea was due to the substantial change in soil pH and rhizospheric nutrient availability such as N, P, K, and Mg, while biomass production and N accumulation were largely attributed to the improved BNF and C accumulation of chickpea in the clay textured soils. The increased growth and nutrient uptake of the following maize crop in the Griffin and Pinedene soils was attributed to high N inputs through BNF and biochar mineralisation, whereas the variation in bacteria communities, chickpea and maize performance in the Fernwood soil was due to the change in rhizospheric soil pH, P, and K. The findings of this study conclude that biochar made from poultry litter is recommended for use as a soil amendment to improve nutrients and soil quality, although caution should be taken when applied at higher rates (40 t/ha) as it may immobilize N or result in high release of toxic elements. Furthermore, incorporating chickpea into existing maize cropping systems of smallholder farmers and using biochar made from poultry litter will help reduce nitrogen input costs by adding residual nitrogen from biochar mineralisation and through BNF, improving soil quality and maize production. On the other hand, biochar made from acacia feedstock will be excellent for use to improve soil organic carbon and water adsorption, but it should be applied months before planting or supplemented with high organic N and P materials when used as a soil amendment to improve nutrient availability, as it may temporary fix N and P in the soil. When using poultry litter or acacia biochar on a poorly buffered loamy sand soil like the Fernwood, care should be taken to avoid over liming, which can cause nutrient deficiency and negatively affect nutrient uptake. Moreover, to improve chickpea and maize performance in sandy textured, highly leached soils like the Fernwood, biochar should be applied regularly or combined with organic materials to improve soil organic carbon to allow the soil to retain and release nutrients for plant uptake.