Soil organic matter and the indigenous nitrogen supply of intensive irrigated rice systems in the tropics

Abstract

Soil organic matter (SOM) has been proposed as an index of N supply in paddy soils although field validations are few. We evaluated the relationship between the indigenous N supply (Ni) of the soil-floodwater system and soil organic carbon (SOC) or total N (Nt) in surface soil of long-term fertility experiments (LTFEs) at 11 sites, in 42 farmer's fields with similar soil type, and in the same field in ten consecutive rice (Oryza sativa L.) crops. The Ni was estimated by crop N uptake from plots without applied N (No plots) under otherwise favorable growth conditions. There was a tight linear correlation between yields and N uptake in No plots and tremendous variation in both parameters among LTFE sites, farmer's fields, and in the same field over time. Correlation between Ni and SOC or Nt explained little of this variation. Factors likely to contribute to the poor correlation were: (1) inputs of N from sources other than N mineralization of SOM in surface soil, (2) degree of congruence between soil N supply and crop demand, which is sensitive to soil drying, length of fallow, crop rotation, and residue management, and (3) differences in SOM quality related to intensive cropping in submerged soil. Better understanding of the processes governing the Ni of tropical lowland rice systems would contribute to the development of crop management practices that optimize utilization of indigenous N resources.

Effects of Sawdust, Forest Soil and Cow Dung Mixtures on Growth Characteristics of Blue Gum (<i>Eucalyptus saligna</i>) Seedlings in South Kinangop Forest, Nyandarua, Kenya

“…It has been reported that soils with less than 0.07% total N will have limited N mineralization potential, while soils with >0.15% total N would be expected to mineralize significant amount nitrogen during the following crop cycle (Kamiri et al, 2013;Cassman et al, 1996;Becker et al, 1994).…”

Increasing Soil Organic Matter Enhances Inherent Soil Productivity while Offsetting Fertilization Effect under a Rice Cropping System

“…Espe et al [18] estimated a linear effect of 1.44 kg/ha for every g/kg increase in SOC for rice on organic soils. However, Cassman et al [30] observed a poor correlation between indigenous N supply and SOC in the rice systems, and attributed the N content to the N inputs from sources other than SOM mineralization, degree of congruence between soil N supply and crop demand, and differences in SOM quality. Obviously, the N supply from SOM mineralization would vary substantially, and it should be incorporated into other fertilization recommendation strategies such as site-specific nutrient management in a given region having similar soil properties, climate, and crop management [18].…”

Nitrogen Transformations in Submerged Soils

“…The total accumulation of N by rice in the absence of fertilizer N has increasingly been used as a fi eld-based measurement of soil N supply, referred to as soil N-supplying capacity (SNSC) or indigenous N supply (INS) (Olk et al, 1999b;Dobermann et al, 2003). Studies in the 1990s, highlighted the high spatial and temporal variability associated with fi eldmeasured INS, based on total accumulation of N by rice in zero-N plots (Cassman et al, 1996b). Field-measured INS is oft en not correlated with SOM, laboratory measurements of soil N availability, or fi eld measurements of inorganic soil N (Cassman et al, 1996b;Adhikari et al, 1999), and even when relationships exist, the measured soil parameters explain only a small portion of the variation in rice yield in zero-N plots (Yadav, 2003).…”