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FORAGES

Soil Nitrogen Mineralization in Mixtures of Eastern Gamagrass with Alfalfa and Red Clover

^a Instituto Nacional de Investigaciones Agrícolas (INIA), CENIAP, Instituto de Investigaciones Zootécnicas, Apartado 4653, Maracay 2101, Aragua, Venezuela
^b Dep. of Agron., Throckmorton Hall, Kansas State Univ., Manhattan, KS 66506-5501

* Corresponding author (whfick{at}ksu.edu)

Received for publication September 6, 2000.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
The amount and rate of soil N mineralization often influences the productivity and persistence of a grass–legume mixture. This research investigated soil N availability in monoculture and binary mixtures of alfalfa (Medicago sativa L.) or red clover (Trifolium pratense L.) with eastern gamagrass [Tripsacum dactyloides (L.) L.] on sandy and clay loam soils near Manhattan, KS. Soil inorganic N and in situ net N mineralization were monitored monthly during the growing seasons of 1996 and 1997. Soil inorganic N was two- to threefold higher with alfalfa, red clover, and gamagrass–alfalfa mixture than with gamagrass in monoculture at the end of 1996. At the midseason of 1997, soil inorganic N was three- to ninefold higher at the clay loam site, but at the sandy site, only alfalfa monoculture was three- to fivefold higher than the other treatments in both years. Soils under alfalfa at both sites in 1997 had the highest cumulative net N mineralized (35–100 kg N ha^-1 yr^-1), followed by the gamagrass–legume mixtures (15–62 kg N ha^-1 yr^-1) and then the gamagrass monoculture treatment (2–15 kg N ha^-1 yr^-1). A high correlation (r² > 0.9, P < 0.05) was found between C/N ratio of the aboveground biomass and the total net N mineralized in the 2nd yr for both sites, suggesting that litter quality is an important driving variable on N mineralization. Our results emphasize the importance of forage legumes in maintaining soil quality and productivity and quality of forage mixtures.

Abbreviations: ANPP, annual net primary production • DOY, day of year • DM, dry matter • PMC, Plant Materials Center • PVC, polyvinyl chloride

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
THE SUPPLY RATE of limiting soil nutrients largely determines the productivity of forage mixtures. Nitrogen is of particular interest because it is usually the most limiting nutrient for forage production, and fertilizer N represents a major variable input cost. Soil N has a variety of pathways for input and outflow and can have important negative environmental impacts when lost from the production system (Steele and Vallis, 1988).

Soil N availability is determined by its mineralization, the microbial conversion of organic N to NH₄ with further oxidation to NO₃. Nitrogen mineralization is regulated by both abiotic factors (e.g., soil moisture, temperature, and texture) and by the supply of above- and belowground litter (Russelle, 1992). Under certain conditions, however, soil microflora can compete with plants for NH₄ and convert it into microbial protein, thus immobilizing the N into an organic, temporarily unavailable form (Steele and Vallis, 1988). In grassland and pasture ecosystems, the mineralization process can be delayed or reduced because of a high C/N ratio or high lignin and polyphenol contents of the litter produced by grasses (Robertson et al., 1993; Jarvis et al., 1996). Low grass quality could cause a deficiency in N available to plants although the N content in soil organic matter might appear adequate (Steele and Brock, 1985; Robbins et al., 1989).

One strategy to overcome low N availability in grass monoculture is to use a forage legume to supply N via biological N fixation process. The various estimates indicate that this process contributes more N for plant growth than the total amount of nitrogenous fertilizers applied to crops each year. On average, it supplies 70 to 80% of the N accumulated by pasture legumes (Danso, 1995). The low cost of inoculation and legume seed make it easy for producers to establish legumes into grass swards to obtain a high quality forage. However, strategic application of N in early spring can be very advantageous and economically feasible for beef and dairy production in some environments (Simpson, 1987). In addition, the relative high quality (i.e., low C/N ratio) of legume litter should lead to reduced net N immobilization in soil under a grass–legume mixture (Thomas, 1992; Cadisch et al., 1994).

Much is known about the effects of N on the productivity of grasses and legumes (Follett and Wilkinson, 1985; Hannaway and Shuler, 1993), but little attention has been paid to the feedback effects of litter quality on N cycling. The nature of the vegetation in a system can have a large effect on the N supply rate of the system, which in turn can strongly influence plant competition and vegetation composition (Wedin and Tilman, 1990; Wedin and Pastor, 1993). Therefore, the selection of a grass and a legume to combine in a mixture should take into account the N-cycling efficiency in such a system.

Eastern gamagrass is a native perennial warm-season grass with high potential as a forage crop because of its high productivity and crude protein content (Faix et al., 1980; Brejda et al., 1997). Eastern gamagrass forage yields responded to increasing rates of N fertilizer at two locations in northern Missouri, yielding up to 13600 kg ha^-1 dry matter (DM) at the maximum N rate applied (336 kg N ha^-1) (Brejda et al., 1997). Inclusion of a legume into stands of this grass may provide the N needed by the gamagrass and maintain adequate levels of crude protein in the forage for animal production.

Few studies have provided data for N mineralization in grass–legume mixtures under field conditions (Hatch et al., 1991). Therefore, our study was designed to compare the effect of a gamagrass–legume mixture on inorganic soil N and net N mineralization with that of gamagrass or legume grown in monoculture on two different soils during the 2nd and 3rd yr of establishment.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Site and Plot Establishment
This study was conducted at two locations in 1996 and 1997. The first site was at the North Agronomy Research Farm, Kansas State University, Manhattan, KS. The soil at this site is an eroded Wymore silty clay loam (fine, smectitic, mesic Aquertic Argiudolls) that is well drained with slow permeability and 1 to 4% slopes. In the upper 10 cm, pH was 5.9 (1:1 soil/water ratio), P content was 37 mg kg^-1, and organic mater content was 38 g kg^-1. The second site was at the USDA Plant Materials Center (PMC), 6 km southwest of Manhattan, KS. Soil at this site is a Haynie very fine sandy loam (coarse-silty, mixed, calcareous, mesic Mollic Udifluvents). The soil had a pH of 7.3 in the upper 10 cm, 18 mg kg^-1 P, and 12 g kg^-1 organic matter content.

Both locations have similar precipitation patterns, with an annual mean rainfall of 835 mm (30-yr average). Rainfall at Manhattan for the study period (April–October) was 680 and 379 mm in 1996 and 1997, respectively. Of the total annual precipitation, 82% normally occurs during April to October. Mean monthly temperatures range from -2.7°C in January to 26.6°C in July. Rainfall and maximum and minimum air temperature recorded at Manhattan for the study period are shown in Fig. 1 and 2.

View larger version (34K): [in this window] [in a new window]   Fig. 1. Daily maximum and minimum air temperature and precipitation recorded at Manhattan, KS during the 1996 growing period.

View larger version (35K): [in this window] [in a new window]   Fig. 2. Daily maximum and minimum air temperature and precipitation recorded at Manhattan, KS during the 1997 growing period.

Inorganic Soil Nitrogen and Net Nitrogen Mineralization
In situ net mineralization of soil N was determined with a soil core technique (Raison et al., 1987). Surface litter was removed, and four polyvinyl chloride (PVC) plastic cylinders (5 cm diam. by 12 cm long) per plot were inserted between rows 10 cm into the soil and covered with lids to prevent leaching losses. Small holes in the exposed areas of the cylinders allowed for gas exchange. At the time that PVC cylinders were placed into the soil, four additional soil cores were also taken nearby. Inorganic N content in these soil samples was considered as the initial N content. At the end of 30 d, soil cores in PVC cylinders were removed and analyzed for inorganic N content, which was considered the final N content. On the same day soil cores were removed, a new set of four PVC cylinders per plot was inserted into the soil for a new incubation period, and additional soil cores were collected for initial N content. Six sets of incubations were obtained per year from mid-April until mid-October. Soils were refrigerated at 4°C until analysis (usually within 1–5 d) and sieved field moist to pass a 6-mm screen. Inorganic N was extracted with 1 M KCl by shaking for 1 h on an orbital shaker at 300 rpm and filtering the supernatant through 0.4 µm of filter paper. Concentrations of NO₃–N and NH₄–N were determined colorimetrically on an Alpkem autoanalyzer (Bull. A303-S021 and A303-S170, Alpkem Corp., Wilsonville, OR). Soil water contents at the beginning and end of the incubation were determined gravimetrically on a subsample dried at 100°C until constant weight (usually 24 to 36 h).

Mean values were calculated from the four cores per plot, and then a mean was calculated from the four replications. All laboratory extractions were conducted separately on soils from each core. Extractable soil N concentration was expressed as mass of N per unit mass of dry soil. Net N mineralization per incubation period was calculated as the difference between the final and initial total extractable N (NH₄–N + NO₃–N). All variables were expressed as mass of N per area unit (kg N ha^-1) using the measured soil bulk density. Bulk density was measured in 1996 with independent soil samples taken from an adjacent area. Undisturbed soil cores (5 cm wide by 12 cm deep) were collected and dried, and bulk density was calculated as the weight of dry soil in a given volume.

Dry Matter Production
Aboveground DM production (yield) per treatment was measured by clipping two 0.5-m² quadrats per plot. Legumes were cut at 6-cm height, and gamagrass and mixtures were clipped at 12 cm. The remainder of the plot was also cut, and most of the material was removed. All plots were uniformly cut in September 1995. For subsequent years, alfalfa was harvested four times per year; the gamagrass–alfalfa mixture was harvested three times per year; and the red clover monoculture, gamagrass–red clover mixture, and gamagrass monoculture twice per year. These frequencies were based on current recommendations for the species used. Harvested material was oven-dried at 65°C until constant weight. Grass and legume biomass in the mixed plots were separated before drying. Because treatments differed in the number of harvests per year, only total annual yields among treatments could be compared statistically.

Dried plant material was ground to pass a 1-mm screen, and subsamples were taken to determine N concentration (g kg^-1) by rapid combustion (850°C), conversion of all N products to N₂, and subsequent measurement with a thermoconductivity cell (Model FP-428, LECO Corp., St. Joseph, MO). Carbon concentration was assumed to be 450 g kg^-1.

Statistical Analyses
Analysis of variance of the effects of site, year, and incubation period was performed using SAS (SAS Inst., 1996). Because interactions of the above factors were significant at P < 0.001, comparisons among treatments were performed separately for each site and year. Sampling dates for inorganic N and incubation periods were considered as subplot, and species were considered as whole plot for each year and site. Means among treatments were compared using t-tests, and LSD were also calculated.

	RESULTS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Soil Inorganic Nitrogen
At Manhattan in 1996, soil from all treatments had similar amounts of extractable N during the first four time intervals, and the amount increased only at the August [day of year (DOY) 238] and September (DOY 269) sampling dates (Fig. 3). During these dates, soils from plots of alfalfa, red clover, and gamagrass–alfalfa treatments had significantly higher extractable N than gamagrass alone or the gamagrass–red clover mixture. In 1997, soil under alfalfa stands had higher levels of extractable soil N during most of the season compared with the other treatments. Red clover plots also had high amounts of inorganic N in the last two periods. Soil under gamagrass had the lowest extractable N in both years for most of the study period. Seasonal patterns of inorganic N for 1997 showed a peak at DOY 167 (mid-June) as well as an increase in extractable N at DOY 227 (August), especially under alfalfa and red clover. A general decline in levels of extractable N for soil under all treatments (except red clover) was observed in the last period (September) for 1997. Overall, higher amounts of inorganic N for alfalfa and red clover were observed in 1997 than in 1996 at Manhattan.

View larger version (38K): [in this window] [in a new window]   Fig. 3. Total inorganic N at Manhattan and Plant Materials Center (PMC) in soils under eastern gamagrass (EGG), alfalfa (ALF), red clover (RC), and their mixtures for two consecutive years. LSD lines indicate significant differences (P < 0.05) among treatments in the same day. Arrows indicate harvest date as follows for 1996: (a) first cut of all treatments; (b) Second cut of alfalfa; (c) second cut of EGG–ALF mixture; (d) third cut of alfalfa; (e) second cut of RC, EGG–RC mixture, and EGG; (f) third cut of EGG–ALF mixture; and (g) fourth cut of alfalfa. For 1997: Same sequence, except for (f) third cut of EGG–ALF mixture and fourth of alfalfa.

Nitrate was the most abundant N form (73 to 99%) at both locations and in both years (data not shown). This higher abundance of NO₃ compared with NH₄ was observed throughout the growing season among all treatments. However, the proportion of NH₄ increased in the last sampling date (September) and in the gamagrass monoculture treatment, particularly in the 2nd yr.

Net Nitrogen Mineralization
Total annual net N mineralization was higher at Manhattan than at PMC in 1997 for all treatments, but no significant differences were observed among treatments between the two sites in 1996, except for the gamagrass–red clover mixture (Fig. 4). For both years and sites, soil under alfalfa in monoculture had the highest total net N mineralization, with a maximum in the second year of study of about 100 kg N ha^-1 at Manhattan. Soils under gamagrass in monoculture and gamagrass–red clover mixture had the lowest annual net N mineralization, particularly in 1997 at PMC where soil with gamagrass decreased >90% compared with 1996.

View larger version (23K): [in this window] [in a new window]   Fig. 4. Total annual net N mineralization at Manhattan and Plant Materials Center (PMC) in soils under eastern gamagrass (EGG), alfalfa, red clover, and their mixtures for two consecutive years. Different letters indicate significant differences (P < 0.05) among treatment means at the same location.

View larger version (35K): [in this window] [in a new window]   Fig. 5. Net N mineralization at Manhattan and at Plant Materials Center (PMC) in soils under eastern gamagrass (EGG), alfalfa, red clover (RC), and their mixtures for two consecutive years. LSD lines indicate significant differences (P < 0.05) among treatment means at the same incubation period. Abbreviations for incubation periods are the first letter of each month, starting with April (A). Arrows indicate harvest date as follows: (a) first cut of all treatments; (b) second cut of alfalfa and EGG–alfalfa mixture; (c) third cut of alfalfa and second cut of EGG, RC, and EGG–RC mixture; (d) fourth cut of alfalfa and third cut of EGG–alfalfa mixture.

Dry Matter Production
In 1996 at both locations, the alfalfa monocultures had the highest yields, whereas the gamagrass monoculture had the lowest yields (Table 1). Dry matter yields of gamagrass in mixtures were lower than yields of the grass in monocultures, but total yields of mixtures were higher than those of gamagrass monoculture. However, yields of mixtures were lower than those of alfalfa or red clover monoculture but similar between gamagrass–red clover mixture and red clover at the PMC. The gamagrass–alfalfa mixture was more productive than the gamagrass–red clover mixture in 1996.

	DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Few measurements of N availability (extractable inorganic N or net N mineralization) have been reported in soils under eastern gamagrass or the legumes studied here (Schertz and Miller, 1972; Piper, 1993). Most of the research on the soil N in forage crop systems has been focused on the effects of N fertilization on DM yields and N production (Hannaway and Shuler, 1993; Brejda et al., 1997). Only a few studies in England have measured gross and net N mineralization directly under field conditions (Geens et al., 1991; Hatch et al., 1991; Gill et al., 1995). In our study, extractable soil N did not exhibit a clear and consistent seasonal pattern during 2 yr although an increase was observed at the end of the growing season, mainly in the 1st yr. Seasonal patterns of soil inorganic N have been reported by Mazzarino et al. (1991) and Turner et al. (1997) for natural grasslands. We observed a more uniform inorganic soil N throughout the early and mid growing season during 1996 in our study. The low soil inorganic N early in the season can be attributed to high rates of plant N uptake at this time, as suggested by Wedin and Tilman (1990), and to the low rates of soil N mineralization. Adequate soil moisture and temperature in the late season of 1996 (Fig. 1) together with slow growth rate and, consequently, low soil N uptake by the plants, may have been the main reasons for the observed increase in soil inorganic N at that time.

Differences among treatments in extractable soil N were more evident in 1997 (Fig. 3), especially at Manhattan. During most of the growing season at this location, soil under alfalfa, red clover, and the gamagrass–alfalfa mixture had higher inorganic N compared with the grass alone. However, at PMC, only soil with alfalfa had significantly higher inorganic soil N compared with the other treatments. Litter quantity and quality coming from a legume or grass–legume mixture compared with a grass monoculture litter has been proposed as the main reason for the differences in inorganic soil N between the two systems (Wedin and Tilman, 1990; Hobbie, 1992). This corresponds with the observations of Virginia and Jarrell (1983) and Mazzarino et al. (1991), who found more NO₃–N in soil under leguminous trees. The low amount of inorganic soil N under gamagrass compared with the other treatments also could have been due to the higher growth and, consequently, greater N uptake observed at both sites in 1997. Piper (1993) also reported low amounts of inorganic N in soils with gamagrass. Though litter quality was not measured in this experiment, we still can address this relationship by taking the C/N ratio of aboveground biomass. It is assumed that N concentration of the aboveground biomass is correlated with N concentration of aboveground residues. This relationship was linear (r² = 0.99, P < 0.001; y = 146.832 - 4.82x) at Manhattan and exponential (R² = 0.99, P < 0.001; y = 225.568 x e^-0194x) at PMC (Fig. 6). No significant relationships between net N mineralization and C/N ratio were found at the two sites in 1996. As reported elsewhere (i.e., Rannells and Wagger, 1996), high C/N ratios of plant residues limit soil N mineralization. Interestingly, the data shown in Fig. 6 reveal not only differences in N mineralization between grass and legumes, but also between legumes. This would indicate that soil N dynamics can vary depending on legume species, so it should be taken into account when selecting a legume to mix with a grass. The most frequently cited reasons (Fox et al., 1990; Hatch et al., 1991) for the greater N mineralization in soils under legumes or grass–legume mixtures have been more easily decomposable material, low C/N and lignin/N ratios, low (lignin + polyphenol)/N ratio, and higher litter contribution from legumes compared with grasses.

View larger version (21K): [in this window] [in a new window]   Fig. 6. Relationship between total annual net N mineralization and C/N ratio for 1997 at Manhattan and at Plant Materials Center (PMC) in soils under eastern gamagrass (EGG), alfalfa, red clover (RC), and their mixtures. Equations are in the text.

Net N mineralization was affected by species, locations, and years. In the first year at both locations, results showed no clear indication of species effect on net N mineralization because only soils under alfalfa had significantly higher net mineralization than the other treatments (Fig. 4). As mentioned before, this could have been partly due to some residual effect of the previous plant residues [wheat (Triticum aestivum L.) at Manhattan and weeds at PMC). The high C/N ratio of roots and residues of wheat (Smith and Sharpley, 1990) on the soil surface might have contributed to a slow net N mineralization in the 1st yr, masking the effects of different plant species on N cycling.

An interesting result was the lower net N mineralization observed at PMC in 1997 compared with the previous year. However, net N mineralization at Manhattan increased in 1997 (Fig. 4). Records for the growing season (April–October) of 1997 show that precipitation was 56% below the long-term average. Because N mineralization is influenced by soil water content (Kladivko and Keeney, 1987), we would expect lower rates of N mineralization and less cumulative N mineralized during the growing season of 1997. This argument could explain the low NO₃–N and total annual net N mineralization registered at PMC in 1997 because the sandy soil of this site has low water-holding capacity. In the same way, the increment of net N mineralization could also be attributed to the soil texture of the site (clay loam), which has higher water-holding capacity. Therefore, although rainfall was less than average, soil moisture would not have been a critical factor that limited microbial activity at Manhattan. Partial support for this hypothesis comes from the soil water content measured during the study (Table 2). Even though soil moisture at Manhattan in 1997 did not reach field capacity (293 g water kg^-1 soil), neither did it reach the wilting point (70 g water kg^-1 soil). Also, soil organic matter protection by clays (Mengel, 1996) at Manhattan during late summer and fall of 1996 and later mineralization in 1997 might have contributed to the observed response.

Regression analysis of net total N mineralization and DM production showed no significant relationship between these two factors at Manhattan; however, a significant relationship (P < 0.05) was obtained at PMC (Fig. 7). This indicates that soil texture influenced this relationship. However, no significant correlations were obtained when all data were combined. Burke et al. (1997) also reported no correlation when soils from a wide range of textures were considered.

View larger version (25K): [in this window] [in a new window]   Fig. 7. Relationship between total annual net N mineralization and biomass production at Manhattan and at Plant Materials Center (PMC) in soils under eastern gamagrass, alfalfa, red clover, and their mixtures in 1996 and 1997. (A) Regression analysis per location and all data considered together, (B) regression analysis for Manhattan in 1996 and 1997, and (C) regression analysis for PMC in 1996 and 1997.

The higher yields obtained at both sites in 1997, despite low precipitation or low mineralization, suggest that other factors—such as pests and diseases, light competition, and physiological and/or morphological adaptations to low soil moisture or low soil nutrient concentration—may affect the relationship between yield and N availability. The relationships among C, N, precipitation, and soil texture are highly interdependent, and separating the effects of these factors on ANPP and N mineralization is difficult. Turner et al. (1997) in tallgrass prairie and Zak et al. (1994) in diverse ecosystems found no clear relationship between ANPP and N availability, suggesting that the controls of soil N transformations and productivity may operate somewhat independently.

In conclusion, this study showed that legume inclusion in a grass stand did have a positive effect on net N mineralization rates, significantly increasing the total amounts of soil N mineralized. These effects were more accentuated in soils under the alfalfa in monoculture and the gamagrass–alfalfa mixture and are probably related to a better quality of the litter produced by this legume compared with red clover. However, some other factors, such as fine-root turnover, root exudates, or nodule decomposition, might have also influenced our results although this study did not evaluate those factors. Clear differences were observed in the 2nd yr after establishment, suggesting that changes in soil N mineralization processes due to changes in plant species composition can occur in a relatively short period.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Contribution no. 99-84-J of the Kansas Agric. Exp. Stn.

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