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SOIL & WATER MANAGEMENT & CONSERVATION

Particulate Organic Matter and Water-Stable Aggregation of Soil under Contrasting Management

USDA-ARS North Central Agricultural Research Lab. 2923 Medary Ave. Brookings, SD 57006

^* Corresponding author (jpikul{at}ngirl.ars.usda.gov).

Soil organic matter (SOM) is important to soil function. The objectives of this work were to determine the effect of cropping rotation and soil management on SOM, components of SOM, and water-stable aggregation (WSA) of soil near the surface. Measurements were made on soil collected from the top 50 mm of seven sites representing contrasts between alternative and conventional management. Management included tillage, crop rotation, native grass pasture, and corn (Zea mays L.) stover removal as silage. At each site, approximately 10 kg of soil was collected from each replication. Soil was separated into six aggregate groups using a rotary sieve. Aggregate size ranges for Groups 1 to 6 were: <0.4, 0.4 to 0.8, 0.8 to 2, 2 to 6, 6 to 19, and >19 mm. Mean weight diameter was calculated using dry aggregate size distribution. Dry aggregate stability, WSA, soil carbon (SC), SOM, fine particulate organic matter (fPOM), and coarse POM were measured on aggregates from each aggregate group. Components of SOM were not uniformly distributed among aggregate groups. Average SC (seven sites) was significantly greater under alternative (31.0 g kg^–1) than conventional (22.3 g kg^–1) management. No tillage (NT) increased fPOM/SOM by 19 and 37% compared with tillage following 4 and 10 yr of NT, respectively. A 5-yr diverse rotation increased fPOM/SOM by 36% compared with monoculture. There was a significant, positive relationship (r² = 0.79) between WSA and fPOM/SOM. Diversity of rotation or reduction of tillage increased fPOM and WSA and this may help to curb soil loss by maintaining surface conditions resistant to erosion.

Abbreviations: ALT, alternative farming practice • CON, conventional farming practice • cPOM, coarse (2.0–0.5 mm) particulate organic matter • DAS, dry aggregate stability • DASD, dry aggregate size distribution • EF, erodible fraction • fPOM, fine (0.5–0.053 mm) particulate organic matter • LOI, loss on ignition • MWD, mean weight diameter • NT, no tillage • POM, particulate organic matter • SC, soil carbon • SOM, soil organic matter • WSA, water-stable aggregation

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