Climate change mitigation through improved soil health: carbon sequestration, nitrogen and phosphorus availability under the push-pull technology as a case study

Abstract

Rebuilding up soil organic carbon (SOC) and by extension nitrogen (N) and phosphorus (P) fertility is a way to mitigating climate change. Push-pull technology could mitigate climate change through sequestering carbon in soils, biologically fixing N and availing P. However, information about this ability is still missing. This information would be used to optimize cropping systems towards sustainable intensification of agriculture. Objectives of this study were to: (1) establish effect of cropping system (push–pull and conventional maize systems), cropping time (years) and agro-ecological zones on carbon stocks, and (2) evaluate impact of cropping system on N and P availability. Three sites (agro-ecological zones); Bondo (LM3 zone) and Siaya (LM2 zone) in Siaya county, and Vihiga (LM1 zone) in Vihiga county, were selected. In each site, farmers who use push-pull were categorized according to how long they had practiced the technology; below 2 years, between 2–5 years, and above 5 years. Five farmers were randomly selected from each category in each site giving 45 farmers (15 from each site). Each farmer had a push-pull plot with maize (Zea mays L.) integrated with desmodium (Desmodium intortum (Mill. Urb.)) (push) and brachiaria (Brachiaria brizantha (Hochst. ex A. Rich.) Stapf. (brachiaria, Mulatto II cultivar) (pull) and one maize cropping system to serve as a control. In these farms, biomass carbon and soil organic carbon (SOC) were assessed. Concurrently, intercrops of maize-common bean (Phaseolus vulgaris L.), maize-crotalaria (Crotalaria ochroleuca G. Don), maize-desmodium, maize-groundnut (Arachis hypogaea L.), maize-cowpea (Vigna unguiculata (L.) Walp.), and maize-green gram (Vigna radiata (L.) R. Wilczek) were compared to maize mono crop (control) in a completely randomized design with four replications in Mbita Point, icipe research station. This experiment was used to study availability of N and P. In both studies (on-farm and on-station), soil sampling was done at 0-15 cm topsoil at around 20 cm from the maize row. Sampling happened immediately after harvesting maize (on-farm) and at 4, 8 and 12 weeks after planting maize (WAP) (on-station). The study covered three consecutive seasons; 2017 long rains (LR), 2017 short rains (SR), and 2018 LR. Push-pull farms stocked between 3.0 ± 0.3 and 4.0 ± 0.4 Mg ha-1 of carbon (C) in crop biomass and between 24.4 ± 2.1 and 37.0 ± 2.6 Mg C ha-1 in the soil. Non-push-pull farms stocked between 1.1 ± 0.3 and 2.1 ± 0.2 Mg ha-1 of carbon in crop biomass and between 19.2 ± 2.1 and 31.1 ± 1.7 Mg soil carbon ha-1. SOC was higher in low rainfall area, Bondo than in Siaya. Farms where push-pull had been practiced for more than five years had higher SOC than those which had push-pull for less than 2 years (P = 0.027). Push-pull increased maize grain yield by 2.33 and 1.77 Mg ha-1 in Vihiga in 2017 LR and SR, and 2.15 Mg ha-1 in Bondo in 2018 LR. An increase in available N and P in maize-desmodium plots compared to maize monocrop plots was observed. Maize grain yield for maize-desmodium was 5.0, 3.1 and 4.3 Mg ha-1 in 2017 LR, 2017 SR and 2018 LR, respectively, compared to 0.5, 0.4 and 1.8 Mg ha-1 for maize monocrop in the three respective seasons. Other intercrops were comparable with maize monocrop. Push-pull technology offers opportunities to mitigate climate change through carbon sequestration in plants and soils in low, medium and high rainfall environments. It also increases availability of N and P and performance of maize. Further studies considering distribution of SOC in the whole soil profile and estimation of N contributed by BNF by desmodium are recommended. In the meantime, adoption of push-pull technology is recommended to farmers.