Soil respiration under different N fertilization and irrigation regimes in Bengaluru, S-India

Rapid urbanization in many countries of the Global South has led to intensification of urban and peri-urban agriculture (UPA) whose effects on the soils’ physical, chemical, and microbial properties have been hardly studied. We therefore investigated the effects of different intensity levels, exemplified by three rates of mineral nitrogen (N) addition and irrigation on CO2 emissions in typical crops during the wet (Kharif) and dry (Rabi) season on a Nitisol in Bengaluru, S-India. Respiration data were collected from 2017 to 2021 in two two-factorial split-plot experiments conducted under rainfed and irrigated conditions. Test crops were maize (Zea mays L.), finger millet (Eleusine coracana Gaertn.), and lablab (Lablab purpureus L. Sweet) under rainfed and irrigated conditions, as well as the vegetables cabbage (Brassica oleracea var. capitata), eggplant (Solanum melongena L.), and tomato (Solanum lycopersicum L.) or chili (Capsicum annuum L.). Carbon dioxide (CO2) emissions were determined using a Los Gatos Research (LGR) multi-gas analyzer whereby under our study conditions CH4, NH3 and N2O were negligible. Measurements were conducted from 7:00 am to 11:30 am and repeated from 12:30 pm to 6:00 pm. Irrespective of irrigation, season, crops and N fertilizer level, CO2 emission rates during afternoon hours were significantly higher (2–128%) than during morning hours. In the irrigated field diurnal emission differences between afternoon and morning hours ranged from 0.04 to 1.61 kg CO2-C ha−1 h−1 while in the rainfed field they averaged 0.20–1.78 kg CO2-C ha−1 h−1. Irrespective of crops, in the rainfed field CO2 emissions in high N plots were 56.4% larger than in low N plots whereas in the irrigated field they were only 12.1% larger. The results of a linear mixed model analysis indicated that N fertilization enhanced CO2 emissions whereby these effects were highest in rainfed crops. Soil moisture enhanced emissions in rainfed crops but decreased them under irrigation where crop-specific CO2 emissions within a season were independent of N application. Soil temperature at 5 cm depth enhanced CO2 emissions in both fields. Overall, higher N and soil temperature enhanced CO2 fluxes whereas effects of soil moisture depended on irrigation.