Mechanical Properties and Organic Carbon of Soil Aggregates in the Northern Appalachians

gregate formation (Horn, 1990; Zhang, 1994). Understanding of mechanical properties of aggregates is cruAggregate properties determine the macroscale structural condicial to explain the macroscale functions of soil for plant tion of the soil. Understanding of impacts of no-till and traditional growth (DeFreitas et al., 1996). agricultural practices on the mechanical properties of aggregates is fundamental to soil management. This study assessed the tensile One of the most useful mechanical properties of agstrength (TS), bulk density ( agg), soil moisture retention (SMR), and gregates is TS (Dexter and Kroesbergen, 1985; Rahimi soil organic C (SOC) concentration of soil aggregates and determined et al., 2000). The TS refers to the force required to the interrelationships among aggregate properties under long-term break an aggregate and is a very sensitive indicator moldboard plow (MP), chisel plow (CP), disk with beef cattle manure of the structural stability of the whole soil (Watts and (DM), no-till with beef cattle manure (NTM), no-till without beef Dexter, 1998). Soils with high aggregate TS offer higher cattle manure (NT), pasture, and forest systems in the North Appalaresistance to mechanical disturbance (Perfect and Kay, chian region. Properties were determined on 1to 8-mm aggregates 1994). The TS is determined on air-dry (Munkholm from 0to 30-cm soil depth. The TS and SMR (0 to 333 kPa) in and Schjonning, 2004) and oven-dry (Watts and Dexter, NTM were higher than those in MP and CP (P 0.01). The SOC 1998) aggregates or in moist aggregates drained at variconcentration for NTM was higher than that for MP, CP, and NT (P 0.01). The agg was 1.35 Mg m 3 in NTM and approximately ous suction heads (Munkholm and Kay, 2002). The TS 1.61 Mg m 3 in MP and CP (P 0.01). Manuring had a positive and is a dynamic measure of the interand intra-aggregate excessive tillage negative impact on aggregate properties. Aggregates bonds and the amount of soil aggregation (Horn and from forest had the lowest TS (63 kPa) and agg (0.99 Mg m 3) and Dexter, 1989). the highest SOC concentration (70 g kg 1), whereas the MP and CP Importance of TS is often overlooked during soil charhad the highest TS (approximately 358 kPa) and the lowest SOC acterization and management (Guérif, 1990). The few concentration (14 g kg 1) in 0to 10-cm depth (P 0.01). Mean agg studies on TS show that TS is highly sensitive to soil and was significantly higher than the density of bulk soil ( b). The logmanagement (Adam and Erbach, 1992; Watts and Dextransformed TS (LogTS) increased with increasing agg and decreased ter, 1998). Intensive cultivation increases TS relative to with increasing aggregate size and SOC. Size, SOC concentration, and uncultivated systems (Munkholm and Schjonning, 2004). agg explained 84% of the variability of LogTS. Long-term ( 35 yr) no-till combined with manuring improved the aggregate properties Tillage at moisture contents above the plastic limit incontrasting with conventionally cultivated systems. creases the aggregate strength compared with aggregates formed by tillage at lower moisture contents. Most of the studies on TS have been conducted on moldboard plowed and grass pasture systems (Watts and Dexter, T macroscale behavior of the soil depends on the mechanical properties of individual aggregates. The 1998; Materechera and Mkhabela, 2001; Munkholm and Kay, 2002). Thus, data on TS from long-term no-till and structural dynamics of the whole soil is defined by the architectural organization and attributes of ever-changother conservation practices are not well documented. Impact of long-term no-till systems on TS and related ing aggregates as the basic units of soil structure development. Aggregates influence root growth and seedling aggregate properties can differ from that of traditional practices because reduced disturbance, improved earthemergence (DeFreitas et al., 1996), soil moisture retention (SMR) and airflow (Watts and Dexter, 1997), and worm activity (Butt et al., 1999), and manuring in notill systems can impact TS differently compared with SOC sequestration and dynamics (Denef et al., 2004). Mechanical properties of aggregates are indicative of moldboard plow systems. Furthermore, TS response to tillage can be soil specific. Benjamin and Cruse (1987) response of the soil system to tillage, compaction, and plant growth. The properties of aggregates may differ reported that soil aggregates of a Canisteo clay loam under paraplow management had lower strength than from those of the whole soil due to the dynamics of agthose under no-till in a Canisteo clay loam. In a Haig silt loam, however, these differences were not significant. H. Blanco-Canqui and R. Lal, Carbon Management and Sequestration Knowledge of differences in mechanical properties among Center, FAES/OARDC, School of Natural Resources, The Ohio State long-term no-till and traditional tillage systems is essenUniv., Columbus, OH 43210-1085; L.B. Owens, USDA-ARS, North Aptial to design proper management practices as stated by palachian Experimental Watersheds, P.O. Box 488, Coshocton, OH 43812; W.M. Post, Environmental Sci. Div., Oak Ridge National Lab., Munkholm and Schjonning (2004). Oak Ridge, TN 37831; R.C. Izaurralde, Joint Global Change Research Aggregates tend to have higher mechanical strength Institute, Univ. of Maryland, College Park, MD 20740. Received 10 than the bulk soil (Semmel et al., 1990) because they comNov. 2004. *Corresponding author ([email protected]). Abbreviations: CP, chisel plow; DM, disk with manure; LogTS, logPublished in Soil Sci. Soc. Am. J. 69:1472–1481 (2005). Soil & Water Management & Conservation transformed tensile strength; MP, moldboard plow; NT, no-till without manure; NTM, no-till with manure; PTFs, pedotransfer functions; b , doi:10.2136/sssaj2004.0356 © Soil Science Society of America soil bulk density; agg , aggregate bulk density; SMR, soil moisture retention; SOC, soil organic carbon; TS, tensile strength. 677 S. Segoe Rd., Madison, WI 53711 USA 1472 Published online August 4, 2005