Leguminous Cover-Crop Residues In Orchard Soils: Decomposition
And Fate Of Nitrogen
Alison M. Berry1, Robert P. McGuinn, Robert L. Bugg,
Ray Eck, Lonnie Hendricks, and Steve Weinbaum1Dept.
of Environmental Horticulture, University of California, Davis
CA 95616
Project Goals
1. To quantitatively trace the fate of legume cover-crop nitrogen in orchard soil using 15N-labeled vetch hay. We will determine when N is released from hay during the almond growing season, how much of the N is available for plant uptake, and what the losses are from volatilization.
2. To test whether "Lana" woollypod vetch, a nitrogen-fixing
legume cover crop, can contribute significant amounts of N to
California almond orchards.
Introduction
Cover crops in orchards can provide several benefits, including improvements in soil structure and water-holding capacity. A potential additional benefit of leguminous cover crops is their capacity to "fix" nitrogen from the atmosphere, and convert it into a usable chemical form. Through decomposition of leaf litter and N release from the hay, nitrogen-fixing cover crops can provide a net input to the soil N budget. Over time, site productivity may be enhanced. In an almond orchard, this could mean that tree N demand can be partially met by application of legume cover crops such as vetch. "Lana" woollypod vetch has been estimated to fix 230 lbs N/acre (Stivers and Shennan 1991).
Nitrogen availability and volatilization. There is a clear need to determine how cover-crop N is mineralized and made available to almond trees under the non-tillage, sprinkler-irrigated regimes that prevail in California. In orchard systems, soil nitrogen availability during the growing season is dependent not only on N input, but also on environmental conditions that influence decomposition and N mineralization, especially temperature and soil moisture. Loss of nitrogen by volatilization of ammonia and/or denitrification of nitrate from cover crop clippings could lead to inefficiency. Our experimental methods will permit us to determine what fraction of N released from a cover crop such as vetch becomes available for uptake by trees, and what fraction is lost by volatilization.
Seasonal pattern of N release from cover-crop hay. Our
experiments will also determine when nitrogen is released from
cover-crop hay during the almond growing season. Total annual
N use by almond trees is estimated to range from 80 to 130 lbs
N/a (Weinbaum et al. 1992). Recent data indicate that patterns
of nutrient uptake in almond trees are conditioned by patterns
of organ nutrient demand (Weinbaum et al. 1984). Major periods
of tree N demand are likely to occur during leaf expansion (March
to mid-April), fruit (pericarp) growth (March-April) and nut fill
(May through July) (see Weinbaum and Muraoka 1986). It is important
to understand whether the timing of cover-crop N release in the
orchard can coincide with periods when almond nutrient uptake
takes place.
Experimental Methods
During summer, 1995, we installed and evaluated the first field experiments to test the method of 15N enrichment of vetch hay and to track the appearance of the 15N label in soil columns. A BIOS-managed orchard near Hilmar, CA, using sprinkler irrigation, was chosen as the study site. Twenty sampling sites were identified within a randomly-chosen row, at midpoints between trees. Irrigation was monitored initially to determine that each site received approximately equal sprinkler coverage.
The vetch was grown in a greenhouse in steam-sterilized soil mix and fertilized with a Hoagland's nutrient solution enriched with 15N-ammonium nitrate (10 atom% 15N). It was grown to flowering, harvested just before field installation, and applied as fresh hay.
Two PVC pipes, 10 in. long by 6 1/4 in. inner diameter, were driven into the ground at each sampling site in the orchard (2 columns X 20 sites). Vetch hay was placed on the top of one soil column, and the second column was left as a control. The hay was placed on aluminum mesh (1/4") in contact with the soil surface, 67 g per column. Anion exchange resin bags were installed at the base of each vetch column per site to measure nitrate leaching. Eight additional columns were installed randomly in the row, and vetch hay without label was applied at the same rate to serve as a non-labeled comparison. Two temperature probes with digital data recorders were placed at one randomly-chosen site--at the soil surface beneath the vetch, or at the soil surface in the control column.
Columns were lifted at five sampling dates: T0 (July
11), and every two weeks after until T4 (September
5). Five columns per treatment per date were analyzed. Each column
was sliced into four, 5-cm fractions. The remaining hay where
present was harvested separately. Subsamples of each fraction
were dried, weighed and ground prior to being analyzed for total
N and 15N content using mass spectrometry. Columns
removed at T1 were sieved and earthworms were collected.
The earthworms were dried, weighed, ground and analyzed for total
N and 15N content. Anion exchange resin bags were refrigerated
for later analysis.
Results
The 15N label enabled us to follow the course of leaf
litter decomposition and N throughput in soil profiles. Statistically
significant enrichment of each fraction in the 10 in. soil column
was detected by T1, two weeks after installation of
the columns and hay (t-test, P=.05), compared to control. Thus
nitrogen from the cover crop hay was already released within the
first two weeks, and detectable with our methods. The amount of
label per soil fraction was not different between T1
and T4. We are still waiting for other mass spectral
data. We have not yet extracted and analyzed the resin bags to
determine a leach fraction.
Conclusions
In the first year of the project, we have developed methods to
enrich vetch for 15N label, and we have installed and
sampled soil columns in the field. We have demonstrated that the
vetch labeling method will be a successful approach for tracing
N throughput. We will also be able to calculate N volatilization
losses. Next year, we hope to follow the fate of cover-crop nitrogen
for a full growing season.
References
Stivers, L. and C. Shennan 1991. J. Production Agric. 4:330-335.
Weinbaum, S. and T. Muraoka. 1986. J. Am. Soc. Hort. Sci. 11:224-228.
Weinbaum, S., R. Johnson, & T. DeJong. 1992. HortTech. 2:112-121.
Weinbaum, S., I. Klein, F. Broadbent, W. Micke & T. Muraoka.
1984. J. Am. Soc. Hort. Sci. 103:516-519.
Cover Crop Research and Education Summaries