Earthworm populations in relation to soil organic matter dynamics and management in California tomato cropping systems

Earthworms are key regulators of soil structure and soil organic matter (SOM) dynamics in many agroecosystems. They are greatly impacted by agricultural management, yet little is known about how these factors interact to control SOM dynamics. This study sought to explore linkages between agricultural management, earthworms and aggregate associated SOM dynamics through a survey of tomato (Solanum lycopersicum L.) cropping systems in northern California. Earthworms and soil samples were collected between February and April of 2005 from 16 fields under one of three types of residue management: (1) tomato mulch – no postharvest tillage and tomato residues left on the soil surface, (2) cover crop – tomato residues tilled in and leguminous cover crop planted, and (3) bare fallow – tomato residues tilled in and soil surface left exposed throughout the winter. Earthworms were collected via hand-sorting and identified to species, while soils were wet sieved to yield four aggregate size classes: large macroaggregates (>2000 mm), small macroaggregates (250–2000 mm), microaggregates (53– 250 mm) and the silt and clay fraction (<53 mm).The combined large and smallmacroaggregate fraction was then fractionated into coarse particulate organic matter (cPOM; 250 mm), microaggregates within macroaggregates (mM; 53–250 mm) and macroaggregate occluded silt and clay (Msc;<53 mm). The earthworms identified in this survey were composed entirely of exotic species and were dominated by Aporrectodea caliginosa. Earthworm abundance was related to residue management, with the tomato mulch systems averaging 4.5 times greater fresh earthworm biomass than bare fallow (P = 0.024). Aggregate stability and total soil C and N also appeared to be influenced by residue management, such that the tomato mulch system displayed significantly greater mean weight diameters than the bare fallow system (P = 0.049), as well as more than 50% greater total soil C and N (P = 0.049 and P = 0.036; respectively). Earthworm biomass was also found to be positively correlated with total soil C (P = 0.009, R = 0.39) and N (P = 0.010,R = 0.039) aswell as theproportionofmacroaggregateC inthecPOM fraction (P = 0.028, R = 0.30). Our findings suggest that residue handling and the associated management practices (e.g., tillage, organic vs. conventional agriculture) are important for both earthworm populations and SOM storage. Although earthworms are known to influence SOM in many ways, other factors appear to play a more prominent role in governing aggregate associated SOM dynamics. # 2008 Elsevier B.V. All rights reserved. avai lable at www.sc iencedi rec t .com journal homepage: www.elsevier.com/locate/apsoil * Corresponding author. Tel.: +1 530 752 6309; fax: +1 530 752 4361. E-mail address: [email protected] (S.J. Fonte). 1 Present address: Division of Natural Sciences, Mission College, 3000 Mission College, Boulevard Santa Clara, CA 95054, USA. 0929-1393/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsoil.2008.10.010 a p p l i e d s o i l e c o l o g y 4 1 ( 2 0 0 9 ) 2 0 6 – 2 1 4 207