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 ϭ 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 aggrega-Green 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 Ϫ1 compost and 45 Mg ha Ϫ1 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 Ϫ1), 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 ϭ 0.65 in 1997) and large (r ϭ longer applied, whether the benefits of amendment per-0.51 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 Growing year Amendment and

Because humic substances are involved in many processes in soils and natural waters, characterization of phosphorus (P) associated with humic substances may shed light on the function of natural organic matter in P cycling and nutrition. In this study, we investigated the spectral features and potential availability of P in the International Humic Substance Society (IHSS) Elliott Soil humic acid standard (EHa), Elliott soil fulvic acid standard II (EFa), Waskish peat humic acid reference (WHa), and Waskish peat fulvic acid reference (WFa) by fluorescence spectroscopy, Fourier-transform infrared spectroscopy (FT-IR), solution 31 P nuclear magnetic resonance (NMR), 3-phytase incubation, and UV irradiation. We observed more similar spectral features between EHa and EFa as well as between WHa and WFa than between the two humic acids or two fulvic acids themselves. Phosphorus in WHa and WFa was mainly present in the orthophosphate form. However, only about 5% was water soluble. After treatment by both UV irradiation and enzymatic hydrolysis, soluble orthophosphate increased to 17% of the P in WHa, and 10% of the P in WFa. Thus, it appears that a large portion of P in Waskish peat humic substances was not labile for plant uptake. On the other hand, both orthophosphate and organic phosphate were present in EHa and EFa. Treatment by both UV irradiation and enzymatic hydrolysis increased soluble orthophosphate to 67% of the P in EHa and 52% of the P in EFa, indicating that more P in Elliott soil humic substances was potentially bioavailable. Our results demonstrated that source (soil vs. peat) was a more important factor than organic matter fraction (humic acid vs. fulvic acid) with respect to the forms and lability of P in these humic substances. This work represents a much more complete characterization of humic substance-bound P than previously reported in the literature, thus providing a comprehensive approach for improved understanding of P cycling in relation to ecosystem function.

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Wood ash is a residual material produced when wood is burned for energy production. It is a useful soil amendment because it raises soil pH and has the potential to supply plant nutrients. The two studies described in this paper were designed to assess the plant availability of P and K in wood ash. Both studies compared nutrient uptake by corn (Zea mays L.) grown in the greenhouse in wood ash—amended soils to nutrient uptake from fertilizer‐amended soils. The wood ashes used were chemically characterized for available P and soluble K content by ammonium citrate extraction, as well as for total P and K. In addition wood ash‐ and fertilizer‐amended soils from the P availability study were extracted with Bray‐1, Olsen, and NH4OAc solutions after harvest. Wood ashes were found to be more similar to conventional P fertilizer materials when the amendments were compared on a citrate‐extractable P basis than when they were compared on a total P basis. For K there was little difference between ash and fertilizer sources, whether they were compared on a total K or a citrate‐extractable K basis. The buffered NH4OAc (pH 3.0) extract was adequate for determining K availability in wood ash—amended soils, but inadequate with regard to P availability in these soils. The buffered solution may have dissolved residual wood ash and released P that was not actually plant‐available. Either the Bray‐1 or the Olsen extract provided a relative measure of P availability that was consistent for both fertilizer‐ and wood ash—amended soils.