Monitoring Changes In Soil Organic Carbon, Moisture Content, Nutrient Balances And Maize (Zea Mays L.) Yield Following Dolichos (Lablab Purpureus (L.) Integration And Fertilizer Application In Maize Systems Of Nairobi County, Kenya

Decline in soil nutrients and organic levels due to continuous cultivation practices combined with erratic rainfall patterns has led to soil fertility decline posing a serious threat to long-term maize (Zea mays L.) production in Nairobi County, Kenya. The current study monitored changes in soil organic carbon, moisture content, nutrient status and maize yield following dolichos (Lablab purpureus (L.) integration and application of fertilizers, as basis for developing sustainable soil fertility management strategies. Field experiments were conducted at the University of Nairobi field station, Kabete Sub-County for two seasons during the mid-March to May 2015 (long rain season; LRS) and October to December 2015/2016 (short rain season; SRS). The experiment layout was a Randomized Complete Block Design with a split-plot arrangement replicated three times. The cropping systems were the main plots; (i) mono-cropping (sole maize) (Zea mays L.), (ii) intercropping (dolichos (Lablab purpureus (L.) /maize) and (iii) rotation (dolichos-maize). The sub-plots were fertilizer types: (i) organic (farmyard manure; FYM), (ii) inorganic (triple superphosphate (TSP) and urea), (iii) integrated fertilizer (FYM +TSP + Urea) and (iv) no fertilizer input (control). Soil moisture, organic Carbon (OC), nitrogen (N), phosphorus (P), and potassium (K) levels were determined at the end of each cropping season. Assessment of ecological sustainability of the technologies being tested was determined by calculating nutrient balances. Soil carbon stocks were also calculated and their changes over a 20-year period projected using Roth-C. Soil carbon (C) inputs were obtained from crop residue and FYM inputs and converted into t C/ha. The C inputs were calculated from grain yield data using a harvest index (HI). The highest levels of soil moisture and organic carbon were respectively observed in maize/dolichos intercrop with application of FYM (31.8% and 2.6%) and FYM + TSP + Urea (30.1% and 2.5%) during the SRS. The same trend was observed in maize/dolichos intercrop with application of FYM and FYM + TSP + Urea in LRS with no significant differences between seasons. Similarly, significantly (P≤0.05) high soil N and P levels were obtained in maize/dolichos intercrop with application of FYM (0.3% and 22.6 ppm; 0.29% and 19.6 ppm) and TSP+FYM+Urea (0.28% and 22.5 ppm; 0.3% and 16.5 ppm) during the LRS and SRS respectively. The soil K levels were significantly (P≤0.05) higher in maize/dolichos intercrop with FYM (1.3 cmol/Kg and 1.8 cmol/Kg) application during the SRS and LRS respectively. Averaged across the two seasons, less negative N balances (kg ha-1yr-1) were obtained in maize/dolichos intercrop with FYM (-9.1) application. Pronounced losses realized in maize/dolichos intercrop with TSP/Urea (-20.1) application. P losses were higher in maize/dolichos with TSP+FYM+Urea (-2.2) and TSP+Urea (-2.4) application. Less negative P balances (kg ha-1yr-1) were obtained in dolichos-maize rotation with the application of FYM (-0.4) and TSP+FYM+Urea (-0.5). Significantly (P<0.5) higher K losses (kg ha-1yr-1) occurred in dolichos/maize intercrop with TSP+Urea (-6.7), dolichos-maize rotation with TSP+Urea (-4.9) and in maize monocrop with TSP+Urea (-4.5) application. Dolichos-maize rotation with FYM application resulted in reduced K losses (-0.2) compared to monocrop with FYM (0.4) and intercrop with FYM (-1.1) application. v Significant (P ≤ 0.05) high SOC (t C ha-1) density and stocks were respectively, obtained in maize/dolichos intercrop with FYM (60.7 and 56.2) and TSP+FYM+Urea (59.6 and 55.2) application compared to sole maize and dolichos-maize rotation during SRS. Higher soil organic carbon (t C ha-1) stocks were obtained in maize/dolichos intercrop with TSP+FYM+Urea (140) application as compared to dolichos-maize rotation (120) and sole maize (110). Over a 20-year period, SOC stocks maintained a significant increase with application of TSP+FYM+Urea and FYM in the order maize/dolichos intercrop, rotation and sole maize system. Maize grain yields (t ha-1) in the SRS were significantly (P≤0.05) higher in dolichos/maize intercrop with application of TSP+FYM+Urea (7.1) and FYM (7.0). Similarly, significantly (P≤0.05) high maize grain yields were obtained in dolichos/maize intercrop with TSP+FYM+Urea (5.2) and TSP+Urea (5.2) during the LRS. Dolichos-maize rotation with TSP+FYM+Urea application resulted in significantly higher dry matter yields (17.9 t ha-1) compared to intercrop with TSP+Urea (19.6 t ha-1) application in the SRS. When compared across the two seasons, soil moisture content, organic carbon and N, P and K levels were consistently high in maize/dolichos intercrop with the application of FYM and TSP+FYM+Urea in SRS. Negative N and P balances were pronounced in maize/dolichos intercrop and dolichos-maize rotation with application of FYM and TSP+FYM+Urea. Significant (P≤0.05) higher dry matter yields were obtained in dolichos-maize rotation with FYM application and higher grain maize yields were realized in intercrop with application of FYM and TSP+FYM+Urea in SRS as compared to the LRS. It is evident that improved soil moisture, organic carbon, nutrient status and carbon stocks in maize/dolichos intercrop with the application of FYM and TSP+FYM+Urea translated into increased maize yields. With the increase in yields, significant nutrient losses were realized. Projected carbon stocks increased in maize/dolichos with continuous application of TSP+Urea and FYM thus replenishing nutrient losses in the long run. Adoption of the best performing technology; maize/dolichos intercrop combined with application of 5 t ha-1 FYM and 60 kg ha-1 TSP+Urea ought therefore to be tapered (in the short run) with prudent nutrient management strategies to minimize nutrient losses through harvested products for system sustainability.