Contribution of pigeon pea (cajanus cajan l. Millsp.) to soil fertility and productivity of maize (zea mays l.) cropping systems in semi-arid Kenya

Abstract

Pigeonpea breeding programs in Kenya have focused mainly on developing high yielding varieties that are resistant to Fusarium wilt and adaptable to a broad range of ecological conditions. However, few studies have evaluated these pigeonpea varieties for soil fertility improvement and contribution to the productivity and sustainability of maize-based cropping systems under a changing climate. A study comprising field and greenhouse experiments was conducted between 2009 and 2013 to: (i) quantify the amount of nitrogen fixed by improved pigeonpea varieties under maize-pigeonpea intercropping systems, (ii) determine the amount of nitrogen (N), phosphorus (P), potassium (K), magnesium (Mg) and calcium (Ca) accumulated by the different components of maize-pigeonpea intercropping systems and its effect on total soil N, available P and exchangeable K, Mg and Ca content, (iii) determine the effect of pigeonpea and maize residues on soil aggregation and soil water content, and (iv) predict the impact of climate change on maize and pigeonpea yields. Field experiments were conducted in Katumani Research Centre using a split-split plot design with three pigeonpea varieties, two cropping systems and three crop residue regimes as the main plot, sub-plot, and sub-sub-plot, respectively. Greenhouse experiments were conducted at Muguga Research Centre where five pigeonpea varieties were screened for biological nitrogen fixation (BNF) and response to Rhizobia inoculation in plastic pots filled with 10 kg of soil and replicated four times in a completely randomized design. Agricultural Production Systems Simulator (APSIM) model version 7.3 was used to predict the impact of climate change on maize and pigeonpea yields. Data collected on total soil N and organic carbon (C), available P, exchangeable K, Mg and Ca, N-uptake, BNF, soil water content, aggregate stability, bulk density and maize and pigeonpea yields were subjected to analysis of variance using GENSTAT statistical software version 14.2. Results showed that all the three pigeonpea varieties fixed 60–70 kgN ha-1, meaning they were all good nitrogen-fixers. However, Mbaazi II fixed significantly (p ≤ 0.05) higher N (70 kg N ha-1) compared to KAT 60/8 (66 kg N ha-1) and Mbaazi I (62 kg N ha-1) when intercropped with maize. Pigeonpea had significantly (p ≤ 0.05) higher N uptake compared to maize; Mbaazi II (84-114 kg N ha-1) absorbed more N followed by Kat 60/8 (29-44 kg N ha-1) and Mbaazi I (20-37 kg N ha-1). Intercropping maize with pigeonpea reduced (p ≤ 0.05) soil organic carbon and total soil N from 1.4 and 0.2% in 2009 to less than 1 and 0.1%, respectively, in 2013. Intercropping maize with long duration pigeonpea (Mbaazi II) and ploughing back 4 t ha-1 of crop residues had no significant effect on available P. However, it increased (p ≤ 0.05) available P from 26 ppm at the start of the study to 50 ppm and 47 ppm in eight seasons under maize-Mbaazi I and maize-Kat 60/8 intercrops, respectively. Exchangeable K, Mg and Ca also declined significantly (p ≤ 0.05). All the maize-pigeonpea cropping systems tested in this study did not improve soil physical properties due to very low soil organic carbon accumulation (< 1%). Instead, they increased soil bulk density beyond the prescribed range for non-restricted plant growth and reduced soil aggregation thereby exposing soils to degradation. However, they did not alter texture of the soils at the study site. Intercropping maize with the three pigeonpea varieties, especially the long duration variety (Mbaazi II), requires more water compared to maize and pigeonpea sole crops. This can be addressed by conserving more water in the profile by ploughing back crop residues. Mbaazi II-maize intercrop offers the best option for farmers in marginal areas like Katumani since it gave the highest maize (1.9 t ha–1) and pigeonpea (1.4 t ha–1) grain yields and produced sufficient maize stover (2.1t ha–1) and pigeonpea stalks (2.9 t ha–1) to plough back and feed the livestock. Simulations showed that maize yields from sole maize crop would increase by 141-150% and 10-23% in 2050 and 2100, respectively. Intercropping maize with pigeonpea will give mixed results on maize yields. Pigeonpea yields will decline by 10-20 and 4-9% by 2100 under CSIRO and CNRM models, respectively, due to the projected 2°C and 11% increase in temperature and precipitation, respectively. Therefore, efforts should be made to develop heat and waterlogging-tolerant pigeonpea varieties to help farmers adapt to climate change and to protect the huge pigeonpea export market currently enjoyed by Kenya.