Fall 1995 (v7n4)
Robert L. Bugg
Article written for Sustainable Agriculture Technical Reviews. 1995
Introduction
Managing cover crops in orchards or vineyards depends in part on understanding their basic biology. This article, presented in two parts, reviews several aspects of cover crop biology. Part I looks at seeds, seedlings, rootzone biology, nutrient uptake, and the fate of cover crop derived nitrogen. Part II (to appear in the next issue of Sustainable Agriculture, Vol.8, No.1) will cover plant community dynamics and allelopathy. Most of the plant species discussed may be used as cover crops or as forage crops in rangeland settings. The issues raised have general applicability to a number of farming systems in California.
Seeds and Seedlings
Williams and Elliott (1960) studied dryland northern Californian populations of crimson clover (Trifolium incarnatum), rose clover (T. hirtum), and subterranean clover (T. subterraneum) with respect to: 1) amount of impermeable seed produced, 2) the rate at which seeds become permeable, and 3) factors causing breakdown of impermeability of rose clover. Crimson clover impermeability declined to a low level in the months after seed maturation. Subterranean clover followed a similar pattern, though slightly delayed. At one site under marine influence, subterranean clover retained a moderate proportion of impermeable seed into autumn. Rose clover maintained a large proportion of hard seed throughout the summer, autumn, and winter months. High temperatures at and slightly above the soil surface were demonstrated to cause breakdown of rose clover impermeability. Rose clover is able to persist better than crimson or subterranean clovers in most dryland northern Californian settings. This characteristic is probably in part due to its prolific production of impermeable seed.
According to Williams (1956), during establishment, the force produced by legume seedlings may be crucial in overcoming the weight of overlying soil and surface crusts. Using glass tubes containing vermiculite and glass rods of known mass, the force produced by seeds of crimson clover, rose clover, subterranean clover, and alfalfa was estimated. Mean forces exerted (in grams, +/ standard error of mean) were estimated as follows. Alfalfa (15.2 +/ 0.5), crimson clover (23.8 +/ 0.2), rose clover (24.1 +/ 0.5), subterranean clover (60.0 +/ 2.9). The force exerted by the seeds was highly correlated (R=0.999) with seed weight, but not so highly (R=0.837) with hydrolyzable carbohydrates, suggesting that other factors may be involved.
Williams (1963) later found that crimson clover varieties showed great variability in the amount of force exerted by seedlings. _Autauga' F.C. 32, 963 and Mississippi selection F.C. 32, 964 showed particularly great forces. These greatly exceeded those of rose clover, _Caliverde' and _Ranger' alfalfa, and _Kenland' red clover. Forces exerted by crimson clover and alfalfa were highly dependent on temperature, with the maximum force attained for the former near 20 C, and for the latter between 25 and 30 C. These patterns suggest that plantings could be timed to correspond with sufficiently warm weather to achieve good emergence of seedlings.
RootZone Dynamics And Nutrient Uptake
Nitrogen
Schomberg and Weaver (1990) found that the addition of the equivalent of 31 kg per hectare of starter nitrogen did not reduce nitrogen fixation by arrowleaf clover, and substantially improved seedling vigor. Starter nitrogen can actually lead to improved nitrogen fixation by legumes.
In a threeyear field trial in Germany, Benkenstein et al. (1990) assessed uptake of nitrate (from sodium nitrate) and ammonium (from ammonium sulfate) by winter cereal rye (Secale cereale cv _Janos') through the growing season, as a function of depth of placement in the soil (40, 60, and 80 cm beneath the surface). Sampling indicated that cereal rye plants absorbed 72 percent of the nitrate derived from the 40cm placement, and 18 percent of that derived from the 80cm placement.
In Germany, Raderschall and Gebhardt (1990) evaluated winter crops of rape (Brassica napus), barley (Hordeum vulgare), and Welsh rygrass (Lolium multiflorum). These respectively accumulated in their aboveground structures 52.1, 36.2, and 22.9 kg of nitrogen per hectare following bell bean (cv _Alfred').
Guiraud et al. (1990) conducted a lysimeter study in which a catch crop of annual ryegrass (Lolium multiflorum) reduced nitrate leaching from 124 kg of nitrogen per hectare to 40 kg of nitrogen per hectare. The percentages of fertilizer nitrogen (as a percentage of total nitrogen) in the water were 19 and 7 percent in covercropped and bare fallow plots, respectively. To a depth of 30 cm, 23 percent of labeled nitrogen was retained in organic form where ryegrass had been incorporated, versus 15 percent under bare fallow.
Jackson et al. (1993) evaluated several potential winterannual nitrogen catch crops for rotation with lettuce in the Salinas Valley of California sown in November and harvested the following March. Particularly promising in terms of nitrogen assimilation were white senf mustard (Brassica hirta cv _Martigena' 200 kg of nitrogen per hectare), cereal rye (Secale cereale cv _Merced' 129 kg of nitrogen per hectare), tansy phacelia (Phacelia tanacetifolia cv _Phaci' 182 kg of nitrogen per hectare), white mustard (Brassica alba 205 kg of nitrogen per hectare), and oilseed radish (Raphanus sativus cv _Renova' 161 kg of nitrogen per hectare). Annual ryegrass (Lolium multiflorum 85 kg of nitrogen per hectare) showed less capacity for nitrogen absorption when used in this temporal niche.
Phosphorus Uptake
Hoffland et al. (1989) grew rape (Brassica napus cv _Jetneuf') in agar plates and nutrient solution with or without phosphorus. Bromocresol purple was included as a pH indicator. Acidification, caused by exudation of citric and malic acids, occurred along a restricted zone about 1.5 cm in length, just behind root tips. Conditions were alkaline along the remainder of the root systems. Concentrations of citric and malic acids were generally lower for plants grown with sufficient phosphorus. Further work will be needed to test whether the acidification observed leads to solubilization of rock phosphate. Potassium, calcium, and nitrate were taken up about twice as rapidly by plants with sufficient phosphate than by plants grown in a phosphorusdeficient medium.
Gardner and Boundy (1983) found that wheat intercropped with white lupin (Lupinus albus) has access to a larger pool of phosphorus, manganese, and nitrogen than wheat grown in monoculture. The former two nutrients were probably mobilized by exudates from the lupin roots, then taken up by the closelyassociated wheat roots.
Based on pot experiments and a literature review, Paynter (1990) concluded that burr medic (Medicago polymorpha) and barrel medic (Medicago truncatula) are not as efficient at absorbing soil phosphorus as is subterranean clover (Trifolium subterraneum).
Annan and Amberger (1989) investigated the ability of buckwheat (Fagopyrum esculentum) to acquire phosphorus. The authors investigated phosphorus uptake, morphological features, and chemical changes in the rhizosphere. Root weight and length, and frequency of root hairs were higher when plants were grown under phosphorus deficiency. Phosphorus uptake rates were only moderate; concentrations of phosphorus in the shoot were high (1.8% of dry weight). Release of P from FePO4 and glucose6phosphate was not due to a buildup of organic acids in the rhizosphere, but to high activities of acid phosphatase, an enzyme produced by buckwheat plants growing in low phosphorus conditions. The following parameters were regarded as important for buckwheat's phosphorus efficiency: 1) a finely divided root system of considerable length, with a high ratio of root surface to root or shoot length; 2) a high storage capacity for inorganic phosphorus, 3) an increased release of protons and FePO4 or MnO2 solubilizing substances by phosphorus deficient plants; 4) a favorable ratio of phosphorus uptake to root mass increase, especially at low phosphorus supply; and 5) a high activity of acid phosphatase in the rhizosphere and the capability to use phosphorus from organic sources.
Fate of Nitrogen Derived from Cover Crops
Sawatsky and Soper (1991) grew field peas using a splitroot procedure. The study suggested that at the time of harvest, 22 to 46 percent of the belowground nitrogen had been shed into the rhizosphere (root zone). Because this nitrogen "rhizodeposition" has not previously been assessed for annual legumes, nitrogen fixation may be underestimated by about 10 percent.
Fox et al. (1990) reported that in order for nitrogen mineralization to occur, nitrogen concentration of incorporated residue must exceed 15 to 25 grams per kg. However, little mineralization will occur if there are high concentrations of lignin or polyphenols. These workers reported a study involving the decomposition of and nitrogen mineralization from residues of six kinds of legumes (alfalfa, round leaf cassia, leucaena, Fitzroy stylo, snail medic, and Vigna trilobata), in which the ratio of lignin+polyphenols to nitrogen content was a good predictor of nitrogen mineralization rate, whereas initial nitrogen concentration of residues was not. After 12 weeks, net nitrogen mineralized ranged from 11 percent from round leaf cassia to 47 percent for alfalfa.
In Terman's (1979) review of ammonia volatilization, he noted that crop utilization of nitrogen from surfaceapplied materials ranges from 30 to 70 percent and may average about 50 percent. Losses of ammonia (NH3) increase with increase in the intensity of drying conditions (higher temperatures, more air movement, and lower humidity), with higher soil pH, with coarsetextured soils of low cation exchange capacity, and with lower initial soil moisture content. Losses are very low if various nitrogen sources are incorporated into the soil or are moved at once into the soil by rain or irrigation.
Janzen and McGinn (1991) reported three experiments evaluating ammonia volatilization from decomposing lentil residue (Lens culinaris Medik.). The first experiment showed that after 56 days, ammonia volatilization from residue left on the soil surface represented five percent of applied nitrogen. Incorporating the residue into the soil stopped nearly all NH3 loss.
In Georgia, on gravelly clay loam and sandy clay loam soils, McVay et al. (1989) conducted a trial of notill corn and sorghum production under various covercropping regimes. Hairy vetch and crimson clover grown as winter cover crops, respectively, replaced on the average 123 and 99 kg per hectare of fertilizer nitrogen. The corresponding aboveground nitrogen contents of cover crop herbage were 128 and 108 kg of nitrogen per hectare. These results suggest efficient cycling of nitrogen in a notill system, with minimal losses due to volatilization of ammonia. Harper et al. (1995) reported that in Georgia for crimson clover preceding sorghum under notill management, volatilization of ammonia was minimal: 323 kg of nitrogen per hectare accumulated in the crimson clover, and an estimated 0.25 kg of nitrogen per hectare (less than 0.1%) was lost by volatilization of NH3.
By contrast, in the Northeast and the Midwest, cyclic wetting and drying may lead to the loss of from onethird to onehalf of the nitrogen contained in surfacemanaged leguminous residues, regardless of the pH of the soil (Sarrantonio and Scott, 1988). Even if field conditions do not lead to substantial volatilization of ammonia, nitrogen availability to the target crop may be delayed in notill systems, as suggested by Lemon et al. (1990) for berseem clover preceding grain sorghum in Burleson County, Texas.
Dr. William L. Hargrove of the University of Georgia has extensive experience in managing notill coolseason annual legumes and in assessing their nitrogen contributions to warmseason annual field crops; h is also an authority on ammonia volatilization. In discussions with the author, Hargrove (personal communication, 1994) stated that ammonia volatilization should be minimal and nitrogen from cover crop residues should become available to trees if understory conditions remain relatively moist during the principal period that the clippings are decomposing. Shading and irrigation, of course, would aid in maintaining the desired moist conditions. Hargrove further indicated that staggered mowing patterns, as often recommended in managing orchard cover crops, will further reduce volatilization by reducing the concentration of ammonia during any one period of decomposition. Rainfall in Georgia averages about four inches per month throughout the year; this corresponds well with the amounts of water applied to sprinklerirrigated almond orchards (91 cm [36 inches] over nine months), and is less than the amounts applied to walnut orchards (122 cm [48 inches] over seven to eight months).
Conclusion
In managing cover crops, many issues must be considered. Here, I have presented a selection of research results on cover crop ecology, emphasizing the dynamics of stand establishment and maintenance, and nutrient cycling. Our knowledge of cover crop ecology is still fragmentary, yet it is progressing rapidly. Thus, these and other results may ultimately be used to develop comprehensive views and management plans. For this to happen most efficiently, farmers, advisors, and scientists should discuss not only the scientific papers themselves, but also the applicability, adaptability, or nonapplicability of the findings within the context of particular farms, or even individual fields. Scientific data like those presented here take on their most important life when they enable farmers to fulfill their aims with minimal expenditure and environmental risk.
References
Annan, C., and A. Amberger. 1989. Phosphorus efficiency of buckwheat (Fagopyrum esculentum). Zeitschift für Pflanzenernährung und Bodenkunde 152:181189.
Benkenstein, H., W. Krueger, and H. Pagel. 1990. Field model test with winter rye on the uptake on N15 labeled nitrate and ammonium from the soil. Archiv von Acker Pflanzenbau Bodenkunde 34:674681
Gardner, W. K., and K. A. Boundy. 1983. The acquisition of phosphorus by Lupinus albus L.: IV. The effect of interplanting wheat and white lupin on the growth and mineral composition of the two species. Plant and Soil 70:391402.
Guiraud, G., J. Martinez, M. Latil, and C. Marol. 1990. Effect of a ryegrass catch crop on the balance sheet of a nitrogen fertilizer. (English translation of French title). Nitrates, agriculture, water. Paris, 78 November 1990.
Fox, R.H., R.J.K. Myers, and I. Vallis. 1990. The nitrogen mineralization rate of legume residues in soil as influenced by their polyphenol, lignin, and nitrogen contents. Plant and Soil 129:251259.
Harper, L.A., P.F. Hendrix, G.W. Langdale, and D.C. Coleman. 1995. Clover management to provide optimum nitrogen and soil water conservation. Crop Science 35:176182.
Hoffland, E., G.R. Findenegg, J.A. Nelemans. 1989. Solubilization of rock phosphate by rape. II. Local root exudation of organic acids as a response to Pstarvation. Plant and Soil 113:161165.
Jackson, L.E., L.J. Wyland, and L.J. Stivers. 1993. Winter cover crops to minimize nitrate losses in intensive lettuce production. Journal of Agricultural Science 121:5562.
Janzen, H.H., and S.M. McGinn. 1991. Volatile loss of nitrogen during decomposition of legume green manure. Soil Biology and Biochemistry 23:291297.
Lemon, R.G., F.M. Hons, and V.A. Saladino. 1990. Tillage and clover cover crop effects on grain sorghum yield and nitrogen uptake. Journa of Soil and Water Conservation 45:125127.
McVay, K.A., D.E. Radcliffe, and W.L. Hargrove. 1989. Winter legume effects on soil properties and nitrogen fertilizer requirements. Soil Science Society of America Journal 53: 18561862.
Paynter, B.H. 1990. Comparative phosphate requirements of yellow serradella (Ornithopus compressus), burr medic (Medicago polymorpha var. brevispina) and subterranean clover (Trifolium subterraneum). Australian Journal of Experimental Agriculture 30:507514.
Raderschall, R., and H. Gebhardt. 1990. Field experiments on the Ndynamics of an aquic plaggen soil cultivated with wintercrops following legumes (Vicia faba L.). (English translation of German title). Zeitschrift für Pflanzenernährung und Bodenkunde 153:7580.
Sarrantonio, M., and T.W. Scott. 1988. Tillage effects on availability of nitrogen to corn following a winter green manure crop. Soil Science Society of America Journal. 52:16611668.
Sawatsky, N., and R.J. Soper. 1991. A quantitative measurement of the nitrogen loss from the root system of field peas (Pisum avense) (sic) grown in the soil. Soil Biology and Biochemistry 23:255259.
Schomberg, H.H., and R.W. Weaver. 1990. Early growth and dinitrogen fixation by arrowleaf clover in response to starter nitrogen. Agronomy Journal 82:946951.
Williams, W.A. 1956. Evaluation of the emergence force exerted by seedlings of small seeded legumes using probit analysis. Agronomy Journal 48:273274.
Terman, G.L. 1979. Volatilization losses of nitrogen as ammonia from surfaceapplied fertilizers, organic amendments and crop residues. Advances in Agronomy 31:189223.
Williams, W. A. 1963. The emergence force of forage legume seedlings and their response to temperature. Crop Science 3:472474.
Williams, W. A., and J. R. Elliott. 1960. Ecological significance of seed coat impermeability to moisture in crimson, subterranean and rose clovers in a Mediterraneantype climate. Ecology 40: 733742.
For more information write to: Robert L. Bugg, University of California Sustainable Agriculture Research and Education Program, University of California, Davis, CA 95616. rlbugg@ucdavis.edu
(DEC.529)
Contributed by Robert Bugg
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