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USDA-ARS, National Forage Seed Production Research Center, Corvallis, OR 97331
* Corresponding author (griffits{at}onid.orst.edu)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: CT, conventional tillage • GDD, growing degree days • MBC, microbial biomass C • MWD, moderately well-drained • NT, no tillage • SOM, soil organic matter • WD, well-drained
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Soil ammonium, directly from applied fertilizer or soil N mineralization, readily binds to soil making these ions relatively immobile. Dispersion of NH4 to waterways, through processes of soil erosion or after being nitrified to NO3, can create water quality problems for fish and aquatic wildlife at certain concentrations, temperature, and dissolved oxygen conditions. Nitrification processes transform NH4 to NO2 and NO3. These soil processes can occur rapidly by soil microbes with adequate soil moisture and temperature under oxidizing conditions. Nitrite and NO3 are relatively mobile, and when not absorbed by plants or microorganisms, can easily move to ground and surface water and adversely affect aquatic wildlife at certain concentrations (Mueller and Helsel, 1996).
Soil drainage, tillage, and crop residue have key influences on mineralization processes, thus it is important to understand how mineralized N are affected by grass seed crops under both conventional and conservation tillage and high residue management conditions on different classes of soil drainage. Currently, there is a trend in western Oregon grass seed production systems to flail-chop postharvest residue and not incorporate it into the soil, and to adopt NT practices. The large amount of soil disturbance of CT accelerates C loss and promotes N mineralization (Stewart and Bettany, 1982). Previous work has demonstrated that NT management increases microbial biomass in agricultural soils (Doran, 1987; Gravatstein et al., 1987; Carter, 1991), as well as the closely related content of active N (Belvins et al., 1977; Lamb et al., 1985; Havlin et al., 1990; McCarty et al., 1995). Compared with NT, CT systems decrease potentially mineralized C and N (Woods and Schuman, 1988) and the soil's ability to immobilize and thus conserve mineral N (Follett and Schimel, 1989). Greater potentially mineralizable N under NT compared to CT, in the upper 7.5 cm of soil, has been associated with a larger microbial biomass (Doran, 1980). A common observation is that the N supplying potential of the soil increases after NT practices have been utilized for a few growing seasons (Franzluebbers et al., 1994), as does the closely related content of active N (McCarty et al., 1995).
Information is lacking in western Oregon about factors that regulate soil N mineralization, including the effects of crop production practices such as tillage. In situ soil N mineralization is rarely measured in agricultural field studies and never in western Oregon. Estimates of N mineralization are commonly based on laboratory incubations conducted under controlled environments and have little relevance to temporal changes in field N mineralization and how it relates to fluctuating crop N sinks. In contrast, methods for measuring soil N mineralization in situ have been routinely used in forest and natural ecosystem studies (Binkley and Hart, 1989; Eno 1960) and can be of great value when used in agricultural systems.
The objective of this study was to determine the effects of CT and NT on crop production in relation to soil mineral-N availability, net N mineralization, and soil microbial biomass C over three cycles of tall fescue or fine fescue seed production. We hypothesized that: (i) during the first year of perennial grass seed crop establishment, using NT, that in situ net N mineralization would be lower compared to rates found for CT soil; (ii) MBC would be greater under NT management; and (iii) following the crop establishment year, perennial grass seed crop soils would return to a more N conserving system, reducing potential mineralized-N losses to the environment.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The WD Marion County soil was representative of soils in the north Willamette Valley foothills compared to the MWD site in Benton County that was typical of some of the southern valley soils. At both locations soil water flow was by percolation. At the Marion site the soil water permeability was moderately slow. At the MWD site soil water permeability was moderate in the upper part of the subsoil and slow in the lower part. These soils were chosen for the study for their extensive presence in the area and contrasting physical and chemical characteristics, which are thought to influence N mineralization processes (Table 1).
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Sites were established in 1992 by the USDA-ARS National Forage Seed Production Research Center as part of a long-term integrated agricultural systems study (Gohlke et al., 1999; Steiner et al., 2006). The study contrasted tillage establishment, crop rotation, and residue management practices. The treatments imposed were direct seeding versus tillage and a subtreatment of seed year. All treatments were replicated with four blocks measuring 560 m2 each. In the spring of 1999, plots were planted with fine fescue (cv. Bridgeport) (WD site) and tall fescue (cv. Hounddog) (MWD site). First, second, and third year stands were included in this study.
Crop management at the WD site consisted of a fallow (2000–2001) and first-year (1999–2000), second-year (2000–2001), and third-year (1999–2000) fine fescue seed crops on soil in continuous 3-yr rotations since 1992. Since our study began after the 1998 to 1999 fallow season, which our first and second production years followed, data presented here for the fallow season were taken in the next time series of the USDA-ARS long-term cropping system research study that began in the fall of 2000 (Gohlke et al., 1999; Steiner et al., 2006). The CT plots were last tilled September 1998 and planted April 1999 and NT plots were last tilled September 1992 and planted April 1999. The third-year CT plots were last tilled May 1995 and planted late April 1997. Since the study began after the first fallow season of 1998 to 1999, data for the fallow season were taken in the next time series of the USDA-ARS cropping system long-term research study which began fall 2000. All plots were fertilized each spring with 134 kg N ha–1, with urea 46–0–0–0.
Crop management at the MWD site consisted of a fallow (2000–2001) and first-year (1999–2000), second-year (2000–2001), and third-year (2000–2001) tall fescue seed crops on soil in continuous 3-yr rotations since 1992. Since our study began after the 1998 to 1999 fallow season, which our first and second production years followed, data presented here for the fallow season were taken in the next time series of the USDA-ARS long-term cropping system research study that began in the fall of 2000 (Gohlke et al., 1999; Steiner et al., 2006). The CT plots were last tilled September 1998 and planted March 1999. The NT plots were last tilled October 1994 and planted March 1999. All plots were fertilized in the spring each year with 134 kg N ha–1, using urea (46–0–0–0).
The in situ buried bag method (Eno, 1960; Binkley and Hart, 1989) was used to quantify net N mineralization using 24 incubations, September 1999 to June 2001, with an average incubation period of 26 d. Three pairs of intact soil cores measuring 50 mm in diameter by 150 mm in length were taken within each plot. One of the core-pairs (Nfinal) was sealed within a self-sealable polyethylene bag (approximately 27 by 28 cm; 1.75-mil thickness) and replaced in its original position in the soil. The bag was covered with loose soil and litter to reduce exposure. Nfinal was collected approximately 26 d later. The other core-pair (Ninitial) was transported to the laboratory sealed in a sealable polyethylene bag for analyses of baseline soil properties. Soil water content was estimated by oven drying at 105°C for 24 h.
Three separate subsamples from the main sample core were extracted with 100 mL of 0.5 M K2SO4, shaken for 30 min on a rotary shaker at 350 rpm and filtered through a Whatman no. 1 filter. The filtrate was then analyzed for NO3–N and NH4–N using a Lachat Quick Chem 4200 analyzer (Milwaukee, WI). An additional set of three soil subsamples were taken for determination of soil moisture by gravimetric methods and determination of soil organic matter (SOM) by combustion.
Soil microbial biomass C (MBC) was determined by CHCl3 fumigation extraction method (Horwath and Paul, 1994). This method measures total organic C (TOC) in extracts of CHCl3–fumigated and nonfumigated soils. Six subsamples from each soil core were used for this analysis. Analyses were performed using a Tekmar-Dorhmann Phoenix 8000 UV-Persulfate TOC analyzer (Mason, OH).
Aboveground crop biomass was collected from three 30-cm2 randomly selected areas in each replicated treatment on 30 Jan., 29 Feb., 30 Mar., 27 Apr., 25 May, 26 June, and 30 Oct. 2000 and 13 Feb., 13 Mar., 13 Apr., 29 May, and 25 June 2001. Biomass was forced-air dried at 70°C for 24 h and then weighed. Plant material was ground using a Tecator Cyclotec 1093 sample mill (Eden Prairie, MN) and analyzed for total N using a PerkinElmer 2400 Series II CHNS/O analyzer (Shelton, CT). Seed yield was determined following pollination during late June. Plots were swathed into windrows. Seed yield from each plot was measured using a Bent Yield Cart and adjusted for clean seed yield.
Calculation of Nitrogen Mineralization and Nitrification
The rate of net N mineralized was calculated by:
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Calculation of Microbial Biomass Carbon
Microbial biomass C in the soil was calculated from the amounts of CO2 C extracted from the fumigated and nonfumigated soil samples:
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Statistical Analysis
Data were analyzed using an ANOVA to determine if there were significant differences between calendar year and, if no differences, data were pooled and an ANOVA was used to determine if there were significant differences between the two treatments for each sample date. Main effects were direct tillage (NT and CT) and site. The subtreatments were plant year (fallow, first year, and second year). All values were expressed as the mean of four replications ± one standard error and significance determined at the P = 0.05 level.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Soil Water
Soil water (w/w) at the WD site was significantly higher (P = 0.05) by 4% compared to the MWD site (Table 2; Nelson, 2003) and was probably due to slightly greater precipitation and, in part, to the higher SOM in the WD soils. The WD site had 3.9% higher (P = 0.05) SOM than soils at the MWD site (Table 1; Nelson, 2003). Soil organic matter can hold up to 20 times its mass in water (Stevenson, 1994). Another factor would be the percentage of clay content; the higher soil clay content the higher the soil's capacity to retain water. Soil at the WD site had 18% more clay than the MWD site (Table 1). Tillage did affect percentage of soil water; tilled plots had significantly (P = 0.05) higher soil water than the direct seeded plots (Table 2).
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No till and CT had no affect (P = 0.05) on seed yield in tall fescue and fine fescue for stand years one and two (Table 3). Third-year seed yields were comparable to first year yields in both species. In the drier year (2000–2001), fine fescue seed harvest was much earlier with respect to GDD and calendar date. These data substantiate previous findings of Steiner et al. (2006).
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Microbial Biomass Carbon
Across tillage treatments, soil MBC was two times higher (P = 0.05) at the WD site than the MWD site (Table 2; Nelson, 2003). Microbial biomass C ranged from 100 to 200 mg C kg–1 soil at the Benton site and 250 to 370 mg C kg–1 soil at the Marion site (Nelson, 2003). This could be explained by differences in percent organic matter and total C (Table 1), as well as differences in soil water holding capacity and aeration. The WD site had twice as much percentage of organic matter and total C, and had higher soil water content, on average, than the MWD site. Moore et al. (2000) found that systems with higher organic matter inputs, and more readily available organic matter compounds, tend to have higher microbial biomass contents.
At the MWD site, soil MBC declined immediately following tillage compared to the NT treatment. At the MWD site, MBC also declined through the season regardless of tillage treatment. This trend continued through the first seed production year. Similarly, at the WD site MBC content declined after tillage but remained at a constant level through the fallow season. The NT/CT MBC ratios remained fairly constant through the season, with the exception of the WD site where MBC content rose at season's end in the first seed production year following NT. By the second and third seed production year, soil MBC level at both sites fluctuated less throughout the season as if it was reaching equilibrium. This was particularly striking at the WD site. These data corroborate reports of other investigators showing that MBC increases under direct seeding (Doran, 1980; Carter and Rennie, 1982; Franzluebbers et al., 1994). The combination of the residue management and NT cultivation used in this study favored a nutrient-rich environment for soil microorganisms to survive and flourish and would account for the observations reported here. Although there is little evidence that enhanced MBC levels in the soil relate to crop productivity, MBC is an overall indicator of soil microbiological activity of the soil environment and may provide information related to soil nutrient processing and crop nutrient acquisition.
Soil Nitrogen
Unlike some poorly drained Willamette Valley, Oregon soils, where NH4 was the dominant N form for much of the winter (Griffith et al., 1997b), NO3 was the dominant N form at the fine fescue (WD) and tall fescue sites (MWD) sites. Common among all seed crop production years and study sites, fall soil NO3 levels declined appreciably from November to January, the months of highest annual precipitation. Early fall soil NO3 levels ranged from approximately 5 to 50 kg N ha–1. This fall–winter soil NO3 flush was facilitated by high rainfall at a time when crop biomass was a weak N sink due to cooler temperatures and shorter days slowing crop growth. During the winter months of January through most of March, soil NO3 concentrations reached their lowest levels, maintaining soil NO3 concentrations of 1 to 10 kg N ha–1. Soil NO3 began to increase in April and May and was attributed to both urea fertilizer application and the stimulation of microbial mineralization of soil organic N. Interestingly, at both sites soil NH4 levels were lowest in the drier year probably resulting from appreciable reductions in mineralization and a more favorable soil environment for nitrification due to a tighter coupling between mineralization and nitrification processes.
Net Nitrogen Mineralization
The WD site had higher rates of net nitrification in the spring compared to the MWD site, where net nitrification rates were greatest in the fall (Nelson, 2003). Annual net N mineralization ranged from 35 to 83 kg N ha–1 yr–1 (average 56.7 kg N ha–1 yr–1) at the Benton site and at the Marion site from 32 to 164 kg N ha–1 yr–1 (average 91 kg N ha–1 yr–1) for NT and 89 to 192 kg N ha–1 yr–1 (average 155 kg N ha–1 yr–1) for CT. These enhanced rates of N processing where significantly (P = 0.05) more pronounced in the normal rainfall year. Net nitrification was significantly enhanced (P = 0.05) by CT (Table 2). The total annual net nitrification was significantly (P = 0.05) greater in tilled soil (Table 3).
Net N mineralization rate varied with soil drainage class. Tilled WD soil resulted in a higher net mineralization rate in the fall following tillage and sowing the previous fall and spring, respectively. Winter 2000 WD first-year CT net mineralization rates were lower than WD NT rates. In contrast, MWD soil resulted in little difference in the rate of net N mineralization among tillage treatments and crop year. Our findings confirm findings of Rice et al. (1987) which showed similar results in contrasting poorly drained and well-drained soils as affected by CT and NT management using a laboratory in vitro intact core mineralization procedure.
At the MWD and WD sites, during the first seed year, net N mineralization rates in the spring, before urea fertilizer application, were greater under NT compared to the CT treatment. Other field studies have shown that spring net N mineralization rates were similar or higher in direct drilled or reduced tilled soils, compared with cultivated soils (Kohn and Cuthberton, 1966; Reeves and Ellington 1974; Stein et al., 1987).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Tillage enhanced N mineralization resulting in more soil NO3 residing in the fall and winter at a time when the crop N sink was lowest. Soil NO3 surplus during this period was subject to major leaching by late fall and winter rainfall. This is especially true for fallow years when an actively growing cover crop was not present. Data here indicated that the magnitude of this response was enhanced in better drained soils. These findings do not support the practice of fall fertilization of western Oregon grass seed crops where the majority of the approximately 1100 mm of precipitation occurs in late fall and winter. Based on soil and plant N dynamics reported here we suggest that growers apply fertilizer N to their perennial grass seed crops in the spring around 400 to 550 GDD in one or split applications depending on the total amount to be applied. Phonologically this period of N application corresponds with the four-leaf stage of development around the time of shoot elongation. The lower recommended N fertilizer rates for fine fescue, 34 to 56 kg N ha–1, compared to tall fescue, 101 to 151 kg N ha–1 (Young et al., 2002), seem reasonable considering that fine fescue seed crops are usually grown on well-drained soils in Willamette Valley, OR, which theoretically would be expected to provided greater amounts of mineralized N than the more poorly drained soils in which tall fescue is often grown. Growers should keep in mind that mineralized N inputs can vary annually with changes in weather, so the optimum N fertilizer rate might need to be adjusted depending on N mineralization inputs. Net N mineralization tests have been developed locally and are available to growers that would approximate N mineralization values for any given soil (Christensen et al., 2003). Data here also indicate that drier years reduce mineralized N inputs; therefore adjustments in N rate may be especially necessary during drier years. It was found that MBC was higher when tillage did not occur. This corroborates, in general, numerous reports of other investigators that MBC was increased under direct seeding (Doran, 1980; Carter and Rennie, 1982; Franzluebbers et al., 1994). Microorganisms take part in the degradation of organic compounds and nutrient cycling into elements that are assimilated by plants.
Data confirm previous findings that fine fescue and tall fescue seed yields were not affected by CT and planting operations, compared to no till direct seeding (Gohlke et al., 1999; Steiner et al., 2006).
Aside from the environmental impact of tillage on soil quality, another incentive for farmers to convert to direct seeding, over conventional crop establishment, would be the proven economic advantage and time saved during conventional field preparation (Gohlke et al., 1999; Steiner et al., 2006). More research is needed under western Oregon climatic conditions to understand N processes as influenced by tillage and soil drainage. Future studies could examine more sites of differing soil drainage class in contrasting climatic years.
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ACKNOWLEDGMENTS |
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Received for publication July 26, 2005.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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