Dynamics of soil nitrogen and phosphorus vis-à-vis nutrient regimes in lowland rice field
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Date
2021-02
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College of Post Graduate Studies in Agricultural Sciences, CAU-Imphal, Umiam
Abstract
An experiment was conducted for partial fulfilment of M.Sc (SSAC) degree in the Kharif season of 2019 to study the nutrient dynamics specially the temporal fate of N and P under lowland rice cultivation at different depths. The experiment was designed in two factors randomized block design with the following objectives of determining the temporal soil solution N and P pool under different nutrient regimes and to assess the relationship of temporal soil solution N and P pools with crop uptake. Three varieties of rice viz. Ranjit (V1), CAU-R1 (V2) and Shasarang1 (V3) were considered and the nutrient regimes considered were (a) Control (T0, no fertilizers), (b) 100% RDF (T1), (c) 100% FYM (T2), and (d)50% RDF + 50% FYM (T3) .Two PVC pipes of 50 mm diameter were installed in each of the experimental plots, at 15 cm and 30 cm depths each for collecting soil solution. Plant and soil samples were collected at 60 and 120 days along with initial soil sampling to determine N and P uptake by the crops. In the experiment only two macro-nutrients, N and P were considered for the studies. N is lost in lowland ecosystem through nitrification and volatilization, denitrification and leaching processes. Losses of P was also reported through drainage water due to percolating water and leaching.
The results revealed that NH4 +-N ranged in between 1.76 mglˉ¹ to 4.89 mglˉ¹ at 0-15 cm depth and in between 1.69 mglˉ¹ to 4.76 mglˉ¹ at 15-30 cm depth of soil solution. NO3 −-N ranged in between 1.51 mglˉ¹ to 4.21 mglˉ¹ at 0-15 cm depth and in between 1.41 mglˉ¹ to 4.27 mglˉ¹ at 15-30 cm depth. P ranged in between 0.55 mglˉ¹ to 1.26 mglˉ¹ at 0-15 cm depth and in between 0.47 mglˉ¹ to 1.16 mglˉ¹ at 15-30 cm depth of solution.
Similar trends of nutrients were seen for SOC, Avl. N and Avl. P. Soil and Solution pH and EC was found to increase with depth of solution respectively. DOC, NH4+-N, NO3 - -N and P in soil solution was generally found to decrease with depth, however in those treatments where inorganic nutrients were applied showed a higher NO3- -N at lower depth of soil solution. In the initial stages of crop growth, solution ammonical and nitrate nitrogen is highly correlated with N uptake but in later stages, though the correlation was positive but it was not significant. However, solution P was significantly correlated with P uptake at both depths till 60 DAT, but at harvest, significance was seen only at lower depths, which might be caused due to deeper penetration of roots in soil system in search of P.
From the study it can be concluded that a considerable amount of N and P were available in the solution phase making them vulnerable to move out of the crop field either through runoff or through sub-surface drainage and leaching. Crops at 60 DAT usually pass through maximum vegetative stage and has tendency for better nutrient uptake. The pattern of uptake at two critical growth stages indicate that N and P availability get reduced towards the maturity stage in solution and a better correlation was observed at 60 DAT than at 120 DAT.
Thus, the null hypothesis is rejected and it can be said that there exists temporal as well as spatial trends in solution N and P pools in the soil system and has a definite positive correlation with crop uptake of nutrients.