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Submitted to: Soil Science
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 6/27/2006 Publication Date: 12/1/2006 Citation: Sainju, U.M. 2006. Carbon and nitrogen pools in soil aggregates separated by dry and wet sieving methods. Soil Science. 171(12):937-949. Interpretive Summary: Soil aggregation has potential benefits on soil moisture status, nutrient dynamics, tilth maintenance, and erosion reduction. Levels of aggregate size distribution and associated C and N pools, however, are influenced by aggregate separation method. Wet sieving method is widely used to determine size distribution and stability of aggregates due to rain drop impact on dry soil causing slaking and surface crusting. Because of rupture of weak aggregates in water, destruction of physical habitat of microbial communities, and exclusion of water soluble C and N in wet sieving, dry sieving of moist soil for determining microbial biomass and activities is getting increased attention. Aggregate size distribution and C and N pools in aggregates were determined by dry and wet sieving in soils collected from four different locations with varying soil properties and cropping systems in eastern Montana and western North Dakota. Aggregate amount was higher in dry than in wet sieving method in 4.75-2.00 mm size class but the amount varied between sieving methods in other size classes. In cultivated soil, no definite trends in C pools in aggregates were observed between sieving methods. In no-till grassland soil, C pools were higher in dry than in wet sieving in <0.25 mm but the trend reversed in >2.00 mm fractions. In all fractions, active and available N pools were 2- to 30-fold higher in dry than in wet sieving, probably due to loss of water soluble N during wet sieving. Both C and N pools, except active C pools, in aggregates were usually higher in <0.25 mm than in other fractions, regardless of sieving methods. Aggregate size distribution and C and N pools determined by dry and wet sieving methods were well correlated, except for active N pools. Because of its rapidity and reliability, dry sieving of moist soil can be used for determining aggregate size distribution and C and N pools compared with more tedious wet sieving method which increases the destruction of physical habitat of microbial communities in aggregates and excludes water soluble C and N pools. The method could be especially useful to measure soil erosion, C and N sequestration, and N mineralization in the soil for the semi-arid drylands of northern Great Plains where rainfall is lower than in subhumid regions and soil erosion is mostly caused by the action of wind. Technical Abstract: Soil aggregation influence conservation and mineralization of C and N but aggregate separation method may influence levels of aggregate size distribution and C and N pools. Aggregate size distribution and soil organic C (SOC), soil total N (STN), particulate organic C and N (POC and PON), microbial biomass C and N (MBC and MBN), potential C and N mineralization (PCM and PNM), NH4-N, and NO3-N concentrations in aggregates separated by dry and wet sieving methods were compared. The PCM, PNM, MBC, and MBN are considered as active pools, SOC and STN as slow pools, POC and PON as intermediate pools, and NH4-N and NO3-N as available pools. Aggregate separation was made in soil samples from 0 to 5 and 5 to 20 cm depths with various properties and cropping systems in northern Great Plains. Aggregate amount was higher in dry than in wet sieving method in 4.75-2.00 mm size class but the amount varied between sieving methods in other size classes. In cultivated soil, no definite trends in C pools in aggregates were observed between sieving methods. In no-till grassland soil, C pools were higher in dry than in wet sieving in <0.25 mm but the trend reversed in >2.00 mm fractions. In all fractions, active and available N pools were 2- to 30-fold higher in dry than in wet sieving, probably due to loss of water soluble N during wet sieving. Both C and N pools, except active C pools, in aggregates were usually higher in <0.25 mm than in other fractions, regardless of sieving methods. Aggregate size distribution and C and N pools determined by dry and wet sieving methods were well correlated, except for active N pools. Dry sieving of moist soil can be used as a rapid and reliable method of separating soil aggregates for determining C and N pools compared with wet sieving which increases the destruction of physical habitat of microbial communities in aggregates and excludes water soluble C and N pools. |
