NITROGEN DYNAMICS IN A RICE-RICE SYSTEM OF UPPER BRAHMAPUTRA VALLEY ZONE (UBVZ) OF ASSAM
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Date
2019-05
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AAU, Jorhat
Abstract
The study investigated mineral N and labile soil organic N dynamics at different growth stages of rice and their distribution within soil depth (0-80 cm) under various combinations of inorganic and organic fertilization in a rice-rice cropping system of UBVZ of Assam. Further partial balance of mineral N and total N stock of the soils were determined. The study was conducted in a long term fertility experiment at Regional Agricultural Research Station (RARS), Assam Agricultural University, Titabar, Jorhat in 2017 taking the selected treatments viz., Control, 100% NPK, 100% NPK + FYM 5t/ha, 50% NPK, 50% NPK+ 50% (FYM) N and FYM 10t/ha for rabi and kharif rice cropping sequence. Soil samples were collected from different depth (0-20, 20-40, 40-60 and 60-80 cm) before transplanting of rabi rice, at active tillering, flowering and physiological maturity stages of rabi and kharif rice and after harvest of kharif rice. Physiochemical properties analysed in the soils collected before transplanting of rabi rice of 2017 was considered as initial. Soil samples of three growth stages of rabi and kharif rice were analysed for mineral and labile soil organic N. Straw and grain samples collected at harvest of rabi and kharif rice were analysed for their N concentration and N uptake. Partial balance of soil mineral N in surface soils (0-20 cm) and total N stock within 0-80 cm soil depth was estimated at the end of the cropping sequence.
The pH of the soils was higher in treatments FYM 10t/ha (5.87), 100% NPK+ FYM 5t/ha (5.72) and 50% NPK+ 50% (FYM) N (5.67) than the control (5.33) but was significantly lower in treatments 100% NPK (5.13) and 50% NPK (5.25). Soil organic carbon (OC) content had built up significantly in all the treatments compared to control. Organic carbon decreased significantly with increased soil depth. The bulk density (BD) of the soils varied from 1.18 (FYM 10t/ha) to 1.36 g cm−3 (control). The highest total and mineral N before transplanting of rabi rice and after harvest of kharif rice was recorded in 100% NPK+ FYM 5t/ha which was significantly higher over rest of the treatments.
In the study of mineral N dynamics, NO3-N and NH4-N were found to be significantly higher in 100% NPK+FYM 5t/ha fertilized plots at the three growth stages in both rabi and kharif rice. The higher NO3-N at active tillering stage was 12.73 and 11.53 mg kg-1 in rabi and kharif rice, respectively, in 100% NPK+FYM 5t/ha fertilization, which gradually decreased at physiological maturity stages of both the rice season (10.70 mg kg-1 in rabi and 9.94 mg kg-1 in kharif). Similar decreasing trend was also observed in NH4-N from active tillering (10.67-17.59 mg kg-1 in rabi and 8.67-16.67 mg kg-1 in kharif) to physiological maturity stages (9.22-16.53 mg kg-1 in rabi and 6.76-14.39 mg kg-1 in kharif). Both NH4-N and NO3 -N showed a distinct decreasing trend with increase in soil depth.
The significantly the highest grain (4.66 and 5.54 t ha−1 ) and straw (6.58 and 6.64 t ha−1 ) yield of rabi and kharif rice and total N uptake were observed in 100% NPK+ FYM 5t/ha treated plots. However, this grain yield was statistically at par with yield in treatments 100% NPK at rabi and FYM 10t/ha and 100% NPK in kharif rice. The soil partial mineral N balance revealed that with application of inorganic fertilizer (100% NPK and 50% NPK) and with no fertilizer application (control) resulted in a negative partial soil N balance. However, application different combinations of organic FYM and inorganic NPK fertilizers as well as 10t/ha FYM alone had lead to positive N balance in soils as well as higher N stock. The highest N balance (18.74 kg ha-1) and N stock (1.51 Mg ha-1) was found in application of 100% NPK + FYM 5t/ha followed by that in application of FYM 10t/ha (3.63 kg ha-1 and 1.33 Mg ha-1, respectively).
The labile soil organic N fractions of the soils viz., microbial biomass N (MBN), particulate organic N (PON) and water extractable organic N (WEON) was decreasing with increase in crop growth from active tillering to physiological maturity stage. The decrease in MBN, PON and WEON in rabi varied from 10.85-16.85 to 9.65-16.03 mg kg-1, 0.21-0.89 to 0.14-0.68 g kg−1 and 6.64-12.43 to 5.31-10.68 mg kg-1, respectively; and in kharif from 8.45-13.04 to 7.28-12.0 mg kg-1, 0.16-0.57 to 0.1-0.48 g kg−1, 4.87-9.49 to 3.48-7.95 mg kg-1, respectively.The labile soil organic N fractions were significantly higher in FYM 10t/ha treated plots which might be due to creation of suitable conditions for microbial growth by FYM. All the labile fractions of soil organic N decreased significantly with increase in soil depth. This might be due to decrease of OC with soil depth.
The N fractions of the soils viz., NO3-N, NH4-N, MBN, PON and WEON showed a significant positive correlation with soil OC and total N content at the three growth stages in rabi and kharif season. Multiple regression analysis of grain yield with various fractions of N showed that NH4-N (R2=0.745) was the main contributor to the grain yield at early growth stage of rabi rice. Whereas, WEON (R2=0.901) was the main contributor to the grain yield in kharif rice. The results of the investigation indicated that the NH4-N and NO3-N were the main contributor to the total uptake of N at three different growth stages in both the rice season. All the fractions of N were significantly and positively correlated with each other (p<0.01). Most of the N mineralized in the soil was expected to occur in the labile N fractions. The positive correlation observed between labile N fractions and mineral N also supported their close interrelation. The multiple regression analysis of various N fractions with total N stock indicated that NH4-N was the main contributor to the N stock of the studied soils under rice-rice cropping system.
The present investigation leads to the conclusion that combined applications of 5 t/ha FYM with 100% NPK at present could be considered as the best nutrient management practices in rice-rice system of UBVZ of Assam in terms of the highest yield, N balance, N stock and sustainable soil health (pH, OC and BD). Moreover, the treatment of organic nutrient management 10t FYM/ha could also produce rice yield statistically at par with 100% NPK + FYM 5t/ha, had positive N balance and was the best treatment for sustainable soil health. Therefore, better organic nutrient management which could produce higher yield than 10t FYM/ha might be the present requirement for the rice-rice system of the UBVZ of Assam.