Effect of water and nitrogen management on aggregate size and carbon enrichment of soil in rice-wheat cropping system § Rojalin Tripathy and Anil Kumar Singh* Water Technology Center, Indian Agricultural Research Institute, New Delhi ± 110012, India Accepted 25 January 2004 Summary A study was carried out on a silty clay loam soil (Typic Haplustept) to evaluate the effect of farmyard manure (FYM) vis-à-vis fertilizer and irriga- tion application on the soil organic C content and soil structure. The fertilizer treatments comprised of eight different combinations of N and FYM and three water regimes. The results indicated that the application of FYM and increasing N rate increased soil organic carbon (SOC) content. Addition of FYM also increased the percentage of large sized water stable aggregates (> 5 mm) and reduced the percentage of smaller size aggregates. This was reflected in an increase in the mean weight diameter (MWD) and improved soil structure. The organic carbon con- tent in macroaggregates (> 1mm) was greater compared to micro-aggregates, and it declined with decrease in size of micro-aggregates. This difference in organic C content between macro- and micro-aggregates was more with higher N dose and FYM treated plots. The effect of resi- dual FYM on MWD and organic C content of the soil after wheat harvest was not significant. The effect was less in deeper layers compared to surface layers of the soil. MWD was significantly correlated with the SOC content for the top two layers. Key words: rice-wheat cropping system / Typic Haplustept / farm yard manure / water stable aggregates / soil organic carbon / mean weight diameter PNSS P117/5B 1 Introduction The rice-wheat system extends to more than 23 Mha in Asia (Ladha et al., 2000). In view of its contribution to the area, production and productivity in this region, this system has assumed immense significance in recent years. However, continuous cultivation of this highly exhaustive cropping sequence in most of the irrigated fertile lands has resulted in the decline of soil physical condition in general, and soil organic carbon (SOC) content in particular. This has led to a reduction in the total factor productivity and raised questions on the sustainability of this cropping system. In an exhaustive cropping system like rice-wheat, both crops respond to higher N dose instead of the present recommended dose for a single crop. One of the questions that need examination is, can a non-flooded water regime in a puddled soil result in an improved soil physical environment for the succeeding wheat crop compared to a continuously submerged soil in addition to savings in water? Application of FYM as organic source of N has been an age- old practice, which fulfills the twin objective of maintaining sustainability and improving soil physical environment includ- ing soil structure. Reduction in bulk density, increase in organic C content and total N content of soil were observed through the application of FYM, along with the recommended dose of N (Sharma and Sharma, 1993). In addition to the above, Katyal et al. (1997) also reported decrease in bulk density, increase in hydraulic conductivity, and improved soil structure through FYM addition. The relationships, between the organic matter content of soil and aggregate size being very complex, are not clearly understood. Oades (1984) and Oades and Waters (1991) proposed a hierarchal concept of soil aggregates consisting of macroaggregates (> 250 lm) and micro-aggregate fractions of 20±250 lm and < 20 lm. Comparison of SOC content in different sized water stable aggregate classes shows that macroaggregates are the source of highly enriched and most labile fraction of SOC (Tisdall and Oades, 1982; Elliott, 1986; Gupta and Germida, 1988; Cambardella and Elliott, 1994). The knowledge of SOC sequestration and turnover also helps in understanding the contribution of soils to green house gas emission (Powlson et al., 1987). Elliott (1986) and Gupta and Germida (1988) reported that crushed macroaggregates released 34±61% more CO 2 -C than the intact macroaggregates, which pre- sented the evidence of physical protection of SOC within macroaggregates. Distribution of organic C in the stable macroaggregates and other options, which may increase the stability of those macroaggregates, thus deserve to be stu- died in detail so as to increase the labile and protected SOC and reduce CO 2 emission. Several workers have investigated the effects of different management practices on the soil organic matter content and aggregation, e.g. the effect of til- lage on SOC content of different size classes was studied by Puget et al. (1995) and Franzluebbers and Arshad (1997); while the effect of cultivation on SOC content of macro- and micro-aggregates was studied by Eillott (1986), Cambardella and Elliott (1994), and Six et al. (1998). However, very scanty literature is available on the effect of FYM on the distribution of SOC content in different aggregate sizes. This study was, therefore, conducted to assess the effect of water, N and the direct and residual effect of FYM addition and their interaction on organic C content and physical properties of soil in the rice-wheat cropping system. ã 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1436-8730/04/0204-1 * Correspondence: Dr. A. K. Singh; E-mail: aks_wtc@yahoo.com § The present study is a part of the Ph D thesis of Dr. Rojalin Tripathy J. Plant Nutr. Soil Sci. 2004, 167, 1±13 DOI: 10.1002/jpln.200321175 1