Organic Amendment and Rotation Crop Effects on the Recovery of Soil Organic Matter and Aggregation in Potato Cropping Systems

der et al., 1994). Water soluble carbohydrates directly stabilize aggregates (Angers and Mehuys, 1989) and Soil structural degradation is common in intensively cultivated have been strongly correlated (r 0.74) with aggregaecosystems due to the depletion of soil organic matter (SOM). We tion (Haynes and Swift, 1990). investigated the mechanisms by which different frequencies of organic amendment application and rotation crops restore C, N, and aggregaGreen manure crops increase soil C inputs, but their tion in gravelly loam soils used for potato production. A single amendeffects on SOM equilibrium and aggregation may occur ment application [FIRST; 22 Mg ha 1 compost and 45 Mg ha 1 beef slowly because background levels of soil C are relatively cattle (Bos taurus L.) manure] did not affect total C in 1996 and inhigh and spatially variable (Robertson et al., 1993, 2000). creased it by 28% in 1997 relative to unamended plots (NONE); light Long-term annual compost and manure applications infraction (LF) C accounted for 56% of this increase. Plots in which crease SOM and structure (Sommerfeldt et al., 1988; amendment was suspended for 1 yr (SASP) following 4 or 5 yr of Gilley and Risse, 2000); however, annual applications annual application had more total C in 1996 (28%) and 1997 (46%) are expensive (Araji et al., 2001) and thus not widely relative to NONE. A green manure crop consisting of oat (Avena sativa utilized in Maine. Applying organic amendments less ‘Porter’), pea (Pisum sativum ‘Trapper’), and hairy vetch (Vicia villosa than annually would be more cost effective if they could Roth) grown in 2-yr rotation with potatoes (Solanum tuberosum L.) reverse the negative effects of potato production. increased soil C in 1997 (25.9 vs. 23.9 g kg 1 ), LF properties in 1996 and 1997, and water soluble carbohydrates (WSC) on several sample Minimizing the frequency of organic amendment apdates relative to an oat rotation crop. Large aggregate (2–6.5 mm) plication depends on predicting the number of applicastability in 1996 and 1997 and medium aggregate (1–2 mm) stability tions necessary to halt declines in SOM and structure in 1997 were increased by FIRST relative to NONE. Total soil C was and, once the decline stops and amendments are no more strongly related to medium (r 0.65 in 1997) and large (r longer applied, whether the benefits of amendment per0.51 in 1997) aggregate stability than LF or water soluble carbohydrate sist. Increases in soil C and total aggregation have been fractions. Compost and manure influences occurred rapidly and were reported in potato cropping systems after a single compersistent, demonstrating that annual applications are not necessary post and manure amendment application (Porter et al., to reverse soil degradation. 1999), but relationships between organic matter pools and aggregation still require elucidation. Effects of organic amendments may last for a century or more on P are Maine’s principal agricultural output abandoned farm land (Compton and Boone, 2000), but and are currently produced on ≈25 000 hectares. the persistence of their influence on intensively cropped Rapid declines in SOM concentration and structural fields with low structural stability is unknown. stability are typical in potato production systems (Saini In this study, we altered organic amendment treatand Grant, 1980) and often lead to erosion (Hepler et ments in a study originally designed to compare unal., 1983; Kachanoski and Carter, 1999). Frequent soil amended potato cropping systems to long-term annually disturbances, including primary tillage, planting, and inamended systems (Porter et al., 1999). Our objectives tensive cultivation for weed control and harvest aerate were: (i) to determine if SOM and aggregation could the soil and break down aggregates, exposing occluded be increased following first-year amendment applicaorganic matter (Lal et al., 1994; Six et al., 1999). Potato tion and, more importantly, if the beneficial effects of crops return only ≈1500 kg ha 1 residue to the soil amendments would be retained after annual applica(Porter and McBurnie, 1996) and the residue has a low tions had ceased; (ii) to determine if a productive green C/N ratio (10:1 for potato shoots) and lignin content, manure crop could enhance active SOM components which increases its decomposition rate (Bending and and aggregation relative to the 2-yr cereal grain rotation Turner, 1999). commonly used in potato systems; (iii) to establish One strategy to increase soil structure is to build orwhether soil aggregation was related to soil C and N, ganic matter pools that facilitate aggregation, such as LF, and/or water soluble polysaccharides in these differLF and WSC. Products of LF decomposition include entially managed soils. polysaccharides and other materials that stabilize aggregates (Six et al., 1998). Light fraction decomposition MATERIALS AND METHODS may also be an important source of plant nutrients (WanSite Characteristics The experiment was conducted at the University of Maine’s A.S. Grandy, W.K. Kellogg Biological Stn. and Dep. of Crop and Aroostook Research Farm in Presque Isle, ME, in 1996 and Soil Sciences, Michigan State Univ., Hickory Corners, MI 49060-9516; G.A. Porter and M.S. Erich, Dep. of Plant, Soil, and Environmental Abbreviations: CONT, amendments applied annually for 5 or 6 yr; Sciences, 5722 Deering Hall, Univ. of Maine, Orono, ME 04469-5722. FIRST, a single amendment application; LF, light fraction soil organic Maine Agric. & Forest Exp. Stn. Publ. No. 2543. Received 9 Mar. matter; NONE, amendments never applied; SASP, amendment inputs 2001. *Corresponding author ([email protected]). suspended after four or five consecutive years of application; SOM, soil organic matter; WSC, water soluble carbohydrates. Published in Soil Sci. Soc. Am. J. 66:1311–1319 (2002).