Nitrogen-fixing Covercrops for California Rice Production (3/4)

G.S. Pettygrove and J.F. Williams
CASE STUDIES IN THE SACRAMENTO VALLEY

Case 1. Sutter County -- Sills Farms Experiment
In a five-year field experiment on a loam soil in Sutter Co., researchers measured the above-ground N content of purple vetch shortly before incorporation in late April each year. Covercrop growth varied from year to year with amount of rain and success in stand establishment. In one year (1993), extremely wet, saturated soil conditions in the late winter and early spring almost completely suppressed covercrop growth, even though an adequate stand had been established. In the other years, N content of the above-ground biomass ranged from 34 to 105 lb N/acre. Including all five years, N content averaged 49 lb N/acre in fall-burned plots and 39 lb N/acre in fall-disced plots (Table 2). In the final year of the experiment, vetch plants exhibited P deficiency. Low soil pH (4.5 - 5.0) contributed to that problem.
Table 2. Purple vetch N content and equivalent fertilizer N value in a continuous rice rotation.
Straw burned in fall Straw disced in fall
Rice crop year Vetch N content N fertilizer replacement Vetch N content N fertilizer replacement
lb N/acre
1990 38 74 16 88
1991 105 108 86 90
1992 57 90 47 60
1993 6 0 10 0
1994 37 70 34 60
5-yr average 49 68 39 60
Data from Sills Farm, Sutter Co. (Rice Research Board annual reports, Project RM-6, 1990-94)

Excluding 1993 data, the fertilizer N replacement value of vetch ranged from 60 to 108 lb N/acre with each pound of N in the vetch above-ground biomass tops replacing 1.4 to 1.5 lb of fertilizer N. Equivalency of vetch N to 100% or more of fertilizer N has been reported by others in California (Case Study 2 and Williams et al., 1968). However, lower fertilizer N equivalencies have also been reported (Williams et al., 1972 described in Case Study 3 below). Possibly in some cases, covercrop N is supplied to rice plants more efficiently than synthetic fertilizer N applied shortly before flooding. The explanation given by some researchers for this relatively greater efficacy of covercrop N is that temporary immobilization of inorganic N during initial stages of decomposition before flooding minimizes losses from denitrification and volatilization (Huang and Broadbent, 1989).
Also, after five years, the top foot of soil on covercropped plots contained about seven percent more organic matter and total nitrogen than the non-covercropped plots. This occurred even though in four of the five years, the non-covercropped plots received nitrogen fertilizer and the covercropped plots did not.
Rice grain yields at the Sills Farms experiment showed a strong nitrogen response. On the straw-burned plots, there appeared to be a non-nitrogen effect. Grain yields on the covercropped plots at the optimal fertilizer N rate exceeded maximum yields on the non-covercropped plots (Figure 1). The yield advantage was small but occurred in four of the five years.

Figure 1. Five-year average yields as influenced by covercropping and rice straw management at the Sills Farms experiment in Sutter County, 1990-94. Straw management and covercropping treatments were repeated annually on the same 0.5 acre plots with six replicates.
Straw breakdown and microbial diversity. After five years, researchers observed faster rice straw breakdown in plots that had a history of rice straw incorporation compared to those where straw had been burned every year (Figure 2). Results indicated that rice straw incorporation, and to a lesser extent covercropping with vetch, enhanced the overall size and diversity of soil microbes (Table 3; and Bossio and Scow, 1995). Although the effects were small, some effect persisted year round and did change straw decomposition rates in the field.
Figure 2. Effect of covercropping and rice straw disposal method on disappearance of straw buried in nylon mesh bags (Unpublished data, D. Bossio). Rice straw disposal methods and covercropping had been in effect for five years before bags were buried.

Table 3. Effect of covercropping with purple vetch and rice straw management method on soil microbial substrate-induced-respiration. Covercropping and straw treatments had been in effect for five years before measurement was made (Unpublished data, D. Bossio. For description of methods, see Bossio and Scow, 1995).
Straw/covercrop practice March 22 April 19
Straw burned mg CO2 g -1 soil hr-1
Vetch covercrop 4.29 bc 2.49 a
No covercrop 3.12 c 2.81 a
Straw spring-incorporated
Vetch covercrop 6.66 a 3.09 a
No covercrop 4.88 b 3.11 a
Values followed by the same letter in a column are not significantly different at the 5% level of significance.

Case 2. Butte County -- Skinner Ranch
A one-year on-farm experiment was conducted in 1989 in Butte County on a clay soil. The field had been burned and disced. Covercrop species were broadcast seeded in mid-October, 1988, on plots replicated six times. Rates of seeding were: Bell bean - 150 lb/acre, purple vetch - 40 lb/acre, Lana vetch - 30 lb/acre. Seed was covered by harrowing. Above-ground covercrop biomass and N content were measured on subplots shortly before incorporation by discing on May 3. Subplots within each covercrop plot received rates of N as ammonium sulfate by drill. M-201 rice was planted and grown with conventional practices, except that the grower withheld N fertilizer on the experiment area.
Biomass yields for the three species ranged from 2,046 to 2,685 lb/acre dry matter. Above-ground N contents in lb N/acre (with standard deviations) were:
Purple vetch 70 ±20
Lana vetch 47 ±6
Bell bean 59 ±10
With no covercrop, grain yields were maximized with 60 lb N/acre (Table 4). On all covercropped plots, grain yield was maximized with no fertilizer N. Thus, the three covercrop species provided N equivalent to about 60 lb N/acre of fertilizer. As in the Sills Farm experiment (Case Study 1), fertilizer replacement value of the covercrop was close to or above 100% of the aboveground N content. For all treatments, rates of N fertilizer above the optimum reduced yield due to lodging and possibly blanking. Maximum yields on covercropped plots appeared to be 2 to 5 cwt/acre higher than the maximum yield on the non-covercropped plots. Rice yields did not differ among the three covercrop species.
Table 4. Effect of covercropping on rice grain N response in Butte Co., Skinner Ranch, 1989.
N fertilizer rate No covercrop Purple vetch Lana vetch Bell bean
lb N/acre grain yield, cwt/acre, 13% moisture
0 67.6 82.8 80.2 83.3
30 73.2 78.1 76.7 79.7
60 78.2 67.2 73.5 77.6
90 70.4 63.1 52.7 59.2
Covercrop treatment means LSD .05 = 2.9 cwt/acre; within N=0 treatment LSD .05 = 7.9 cwt. Maximum yields for each covercrop treatment are highlighted.

Case 3. Butte County -- Rice Experiment Station (Williams et. al, 1972)
A five-year field experiment was conducted at the Rice Experiment Station in Biggs on a Stockton clay soil to compare straw incorporation methods and to determine the effect of covercropping with purple vetch. Subplots were fertilized annually with 0, 40, 80, and 120 lb N/acre as ammonium sulfate drill applied. Five-year average grain yields showed no measurable difference between rice straw burning and incorporation at any level of fertilizer or vetch N. Vetch covercropping had a positive effect on yield equivalent to about 40 lb N/acre (Table 5). The effect was similar on straw-burned and straw-incorporated plots (not shown). The researchers did not report purple vetch seeding rate, planting method, or vetch biomass and N production. Generally, vetch grew poorly and supplied only about 25% of the N needs of the rice crop.
Table 5. Effect of covercropping with purple vetch on rice grain yield averaged over five years and across straw-burned and straw-incorporated treatments.
N applied to rice Purple vetch No covercrop
lb/acre cwt/acre, 14% moisture
0 43 27
40 51 41
80 56 52
120 54 57
Maximum yields for each treatment are highlighted.

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Modified: 4 Dec 1997 Comments to jayoung@ucdavis.edu