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ARS Home » Plains Area » Bushland, Texas » Conservation and Production Research Laboratory » Soil and Water Management Research » Research » Publications at this Location » Publication #242948
Title: Soil total carbon content, aggregation, bulk density, and penetration resistance of croplands and nearby grasslands

Author
item UNGER, PAUL - Retired ARS Employee

Submitted to: Soil Science Society of America Special Publication Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 8/1/2008
Publication Date: 6/1/2009
Citation: Unger, P.A. 2009. Soil total carbon content, aggregation, bulk density, and penetration resistance of croplands and nearby grasslands In:Lal R.,Follett R., editors.Soil Carbon Sequestration and the Greenhouse Effect.2nd edition. Madison, WI:Soil Science Society of America, Inc. p. 123-140.

Interpretive Summary: Soil organic matter content, a part of soil total carbon, usually decreases when native grassland (NGL) is used as cropland (CL). This often causes soil degradation. When grass is reestablished on CL, organic matter usually increases, which improves soil conditions. In this study, effects of different land uses [CL, NGL, and Conservation Reserve Program (CRP) land (CRPL)] on soil total carbon content, aggregation, bulk density, and penetration resistance were determined. Samples were taken at 11 locations in the Texas High Plains part of the southern Great Plains. Penetration resistance was measured at the locations. Total carbon contents (Mg ha**-1) at 0- to 2-cm depths averaged 2.7 on CL, 5.4 on CRPL, and 7.6 on NGL. Trends were the same deeper in the soil. Reestablishing grass on CL increased carbon contents, but over 10 years are needed to raise the contents to amounts found on NGL. The mean weight diameter of water-stable eaggregates and stability of 1- to 2-mm aggregates in water were positively related to total carbon. The percentage <0.25-mm diameter of aggregates that were stable in water was negatively related to total carbon. These effects on aggregates show the importance of keeping soil total carbon contents at a high level, thereby providing soil conditions that result in using water more efficiently. Land use had little effect on soil bulk density and penetration resistance, and relationships between them were not significant. Few relationships between soil water content and penetration resistance were significant, probably because conditions differed widely at the different locations. Establishing grass on CL reverses soil degradation, but crop production is lost. Hence, practices that reduce soil carbon losses should be used to reduce CL degradation.

Technical Abstract: Converting native grassland (NGL) to cropland (CL) decreases soil organic matter contents (components of soil total carbon contents, STCCs), which often leads to soil degradation. Reestablishing grass on CL generally increases soil organic matter, which improves soil conditions. This study was conducted to determine effects of land uses [CL, NGL, and Conservation Reserve Program (CRP) land (CRPL)] on STCC (mainly organic carbon), aggregation, bulk density, and penetration resistance. Cropland, NGL, and CRPL sites were sampled at 11 locations in the southern Great Plains. Penetration resistance was measured at the sites. Mean STCCs at 0- to 2-cm depths were lowest on CL, intermediate on CRPL, and highest on NGL (2.7, 5.4, and 7.6 Mg ha**-1, respectively). Trends were the same at greater depths. Reestablishing grass on CL resulted in increased STCCs, but achieving contents comparable to those on NGL will require more than 10 years. Water-stable aggregate mean weight diameter and water-stability of 1- to 2-mm aggregates were positively related and percentage of <0.25-mm water-stable aggregates was negatively related to STCC. The aggregate results indicate the importance of high STCCs for maintaining conditions conducive to using precipitation and irrigation water efficiently. Land use affected bulk density and penetration resistance slightly, but relationships between them were not significant. Few relationships between soil water content and penetration resistance were significant, probably because of widely diverse conditions involved. Establishing grass on CL reverses soil degradation, but crop production is lost. Hence, practices that minimize STCC losses should be used to minimize CL degradation.