Carbon: nitrogen dynamics in acid sulphate and acid saline rice soils of Kerala

Abstract

The present study was undertaken to unravel the chemistry of Carbon: Nitrogen dynamics in submerged acid sulphate and acid saline soils, to identify the labile fractions of these elements contributing to soil fertility and to modify the organic carbon based fertility ratings for nitrogen recommendation in Kale lands. Forty-five representative soil samples from 4 different rice growing acid saline and acid sulphate soils of Kerala were collected and characterized for pH, EC, OC, total carbon, total nitrogen, available nutrients (N, P, K, Ca, Mg, S, Fe, Cu, Mn, Zn and B) and microbial biomass carbon. The soil samples were analysed as such after sampling on wet basis and the results were expressed on moisture free basis to have uniformity. The organic carbon status of the soils varied from 0.81 to 7.58 per cent. Soils from Kaipad recorded the lowest and soils from Vechoor Kari of Kuttanad recorded the highest value of organic carbon. The total nitrogen ranged from 0.05 per cent in upper Kuttanad to 0.42 per cent in Vechur Kari. The highest available nitrogen content of281.38 kg ha' was recorded in sample from Vechoor Kari and the lowest of 19.84 kg ha in Purakkad Kari. The C:N ratio varied from 13:1 to 24:1. Widest C:N ratio was recorded in soils of Upper Kuttanad and the lowest in soils from Thrissur Kale. The organic carbon was significantly and positively correlated with total nitrogen and available nitrogen status. Total nitrogen was the single most independent factor explaining 94 per cent variability of organic carbon. Soil samples were subjected to fractionation studies (both physical and chemical) to quantify the carbon and nitrogen that is associated with different inorganic and organic constituents in soil. In physical fractionation, soil carbon and nitrogen preferentially recovered from the sand, silt and clay size fractions were estimated. Of this, carbon recovered from clay size fraction was the dominant independent variable that explained 64.6 per cent variability of organic carbon. The different chemical carbon fractions studied were water soluble carbon (WSC), hot water extractable carbon (HWEC) and permanganate oxidizable carbon (POC). The water soluble carbon being derived from completely decomposed organic matter was not associated with nitrogen in soils. The HWEC being the most labile pool of carbon had significant influence on mineralisation process thereby contributing to total and available nitrogen content. The permanganate oxidizable carbon being a stabilized pool might not undergo further decomposition to release nitrogen and hence, its contribution to available pool was negligible. Among the organic pools of nitrogen, the total hydrolysable nitrogen contributed significantly to mineralizable N. Among the inorganic fractions of nitrogen, ammoniacal nitrogen was contributing more to the available pool of nitrogen than nitrate nitrogen fraction probably because of high solubility and losses of latter by leaching. A field experiment was conducted to investigate the response of rice to different levels of nitrogen in Adattu Kole with an initial C:N ratio of 20:1. The treatments with increased levels of nitrogen based on C:N ratio (treatments Ts-TlO) produced significant effect on plant height, number of productive tillers, number of grains per panicle, straw yield and grain yield. The total nitrogen content both in soil and plant were significantly influenced by higher doses of nitrogen fertilizers prescribed as per the C:N ratio. Among the carbon fractions, hot water extractable carbon contributed more to the mineralizable pool than water soluble carbon. The direct effect of total hydrolysable nitrogen on total and available nitrogen was very high. Ammoniacal nitrogen being a dynamic and time dependent variable, though contributing significantly to available N content, its effect on total nitrogen was negligible. This was in conformity with the results of experiment in characterization of soil samples collected from 45 locations. The maximum grain yield of 8.22 Mg ha" was recorded in the treatment where nitrogen was applied based on C:N ratio (wet analysis). An increase of 1.15 Mg ha of grain yield was recorded over the treatment where soil test based fertilizer recommendation was applied. The highest straw yield of 17.47 Mg ha was recorded in treatment where nitrogen applied was double that of C:N ratio based recommendation. The highest net return was obtained in treatment where nitrogen was applied as per the C:N ratio in soil.