two ways. First, fertilizers can increase crop production, leading to higher inputs of crop residues and more WSA Agricultural practices that alter the soil organic matter (SOM) than in unfertilized soils (Campbell et al., 2001). Second, content are expected to cause changes in soil stability and aggregation. soils amended with organic fertilizers such as legume The objective of this study was to evaluate short-term (Ͻ2 yr) changes in water-stable aggregates (WSA) in a silt-loam soil under different green manure or animal manure tend to have a higher management regimes. The interactive effects of tillage (no-till and input of OM and more WSA than soils that receive conventional tillage), crop rotations (continuous corn, corn-soybean other types of fertilizer (Angers and N'Dayegamiye, rotation) and composted cattle manure applications [0, 15, 30, and 1991; Sun et al., 1995; Haynes and Naidu, 1998; Aoyama 45 Mg (wet weight) ha Ϫ1 ] on WSA were assessed in a factorial (tillet al., 1999). age ϫ crop rotation) split plot (compost) experiment. The proportion Although the separate effects of tillage, cropping sysof WSA Ͼ4 mm was greater in compost-amended than unamended tems, and fertilizers on aggregation have been docusoils within 1 yr, and the mean weight diameter (MWD) of aggregates mented, there is limited information on the ways that increased with increasing compost application rates. By the second these factors interact to affect aggregation. Bissonnette year of the study, no-till soils under continuous corn and the soybean et al. (2001) examined the interactive effects of eight phase of the corn-soybean rotation had more WSA Ͼ4 mm and a greater MWD than any crop rotation in conventionally tilled soils. management systems that included continuous barley Increasing the C input to soil increased the MWD of aggregates. The and a 3-yr barley-forage rotation, moldboard and chisel MWD of aggregates was related to the C content of soils under noplowing in the fall, and fertilization with liquid dairy till, but not conventional tillage, suggesting more physical stabilization manure or mineral fertilizers. Chisel-plowed soils under of organic matter (OM) in no-till than conventional tillage agroecosysthe barley-forage rotation that received liquid dairy tems. Our findings indicate rapid improvements in aggregation of a manure had among the highest annual C input, which silt-loam in the first 2 yr after compost application and the adoption led to a higher soil C content and a greater MWD of of no-tillage practices. aggregates in this system than other management systems. Bissonnette et al. (2001) concluded that management systems that increased the soil C content also Yield Estimated NPP † Estimated C input ‡ Tillage System Crop Rotation

The importance of spring thaw nitrous oxide (N2O) fluxes to the total N2O emission budget in cold climates has been recognized recently. Two mechanisms have been proposed to explain the burst in N2O fluxes due to soil freezing and thawing: enhanced microbial activity due to increased nutrient availability at spring thaw, and release of N2O trapped at depth during winter. The objective of this study was to determine whether increased surface N2O fluxes were due to physical release at spring thaw of N2O accumulated all winter at depth in the soil profile, or whether fluxes were due to rapid N2O production in the surface layer during the thaw process. Micrometeorological flux measurements and a chamber method applied to in situ soil columns receiving 15N tracer were used in Ontario, Canada during winters of 2003 and 2004. Labeled K15NO3 fertilizer (60% excess 15N) at the rate of 100 kg N ha−1 was applied to two layers, that is, surface layer 0 to 5 cm (SL) and deep layer 12 to 17 cm (DL) in nondisturbed soil columns placed in the field during the winter. The burst in N2O fluxes from the soil surface measured by both methods occurred within the same period of soil thawing. Denitrification was the main mechanism responsible for N2O production, and conditions conducive to N2O and N2 production occurred both in the SL and DL during thawing. Despite high 15N2O concentrations at depth, the burst in N2O fluxes from DL soil columns were 1.5 to 5 times lower than that from SL soil columns as more N2O from DL was converted to N2 before diffusing out of the soil profile. Comparison of N2O fluxes originating from SL and DL soil columns indicates that the source of N2O burst at spring thaw is mostly ‘newly’ produced N2O in the surface layer, and not the release of N2O trapped in the unfrozen soil beneath the frozen layers.

Phosphorus movement in subsurface flow from agricultural soils can be a significant pathway contributing to eutrophication of surface waters. Our study aimed to evaluate several environmental and agronomic soil P tests as indicators of dissolved reactive P (DRP) concentrations in soil-column leachate from Ontario soils. Undisturbed soil columns were collected from six major soil series, with 10 sites of each to quantitatively cover a wide range of soil test P (STP) or degree of P saturation (DPS). Split-line models described the relationships (P < 0.001) between leachate DRP concentrations and the values of In(STP) and In(DPS), with a greater slope observed above the change points than below them. Among the tested soil P measures, water-extractable P (WEP), Mehlich-3 P/(Mehlich-3 Al + Fe) (DPS^^3-1), and Mehlich-3 P/Mehlich-3 Al (DPS^,3-2) had the strongest overall relationships with leachate DRP concentration. Ontario soils were grouped into no-risk, low-risk, medium-risk, and high-risk categories based on the conditional probability of yielding leachate DRP > 0.1 mg P L"^ at a given STP as measured by WEP and Olsen P or a given DPS as measured by DPS^^j-i and DPS,^.j-2. While the Olsen P test is most commonly used for agronomic calibration in Ontario, DPS^3-2 provided the best indicator of leachate DRP concentration from Ontario soils. Regardless of the test method used, these numeric criteria could be combined with site hydrology and P management practices for a more comprehensive soil P loss assessment.Abbreviations: DPS, degree of phosphorus saturation; DPS^^,-1, Mehlich-3 phosphorus/ (Mehlich-3 aluminum + iron); DPS^¡-2, Mehlich-3 phosphorus/Mehlich-3 aluminum; DPS^3-3, Mehlich-3 phosphorus/Mehlich-3 calcium; DRR dissolved reactive phosphorus; FeO-P, iron-oxide-coated filter paper strip phosphorus; STP, soil test phosphorus; WEP, water-extractable phosphorus.

Phosphorus (P) loss from agricultural land in surface runoff can contribute to eutrophication of surface water. This study was conducted to evaluate a range of environmental and agronomic soil P tests as indicators of potential soil surface runoff dissolved reactive P (DRP) losses from Ontario soils. The soil samples (0- to 20-cm depth) were collected from six soil series in Ontario, with 10 sites each to provide a wide range of soil test P (STP) values. Rainfall simulation studies were conducted following the USEPA National P Research Project protocol. The average DRP concentration (DRP30) in runoff water collected over 30 min after the start of runoff increased (p < 0.001) in either a linear or curvilinear manner with increases in levels of various STPs and estimates of degree of soil P saturation (DPS). Among the 16 measurements of STPs and DPSs assessed, DPS(M3) 2 (Mehlich-3 P/[Mehlich-3 Al + Fe]) (r2 = 0.90), DPS(M3)-3 (Mehlich-3 P/Mehlich-3 Al) (r2 = 0.89), and water-extractable P (WEP) (r2 = 0.89) had the strongest overall relationship with runoff DRP30 across all six soil series. The DPS(M3)-2 and DPS(M3)-3 were equally accurate in predicting runoff DRP30 loss. However, DPS(M3)-3 was preferred as its prediction of DRP30 was soil pH insensitive and simpler in analytical procedure, ifa DPS approach is adopted.