Simulating dairy liquid waste management options as a nitrogen source for crops

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Abstract

Large scale dairy operations are common. In many cases the manure is deposited on a paved surface and then removed with a flushing system, after which the solids are separated, the liquid stored in ponds, and eventually the liquid applied on adjacent crop land. Management of liquid manure to maximize the fertilizer value and minimize water quality degradation requires knowledge of the interactive effects of mineralization of organic N (ON) to NH4+, crop uptake of mineral N, and leaching of NO3 on a temporal basis. The purpose of the research was to use the ENVIRO-GRO model to simulate how the amount of applied N, timing of N application, ON mineralization rates, chemical form of N applied, and irrigation uniformity affected (1) yields of corn (Zea mays) in summer and a forage grass in winter in a Mediterranean climate and (2) the amount of NO3 leached below the root zone. This management practice is typical for dairies in the San Joaquin Valley of California. The simulations were conducted for a 10-year period. Steady state conditions, whereby an equivalent amount of N applied in the organic form will be mineralized in a given year, are achieved more rapidly for materials with high mineralization rates. Both timing and total quantity of N application are important in affecting crop yield and potential N leaching. Major conclusions from the simulations are as follows. Frequent low applications are preferred to less frequent higher applications. Increasing the amount of N application increased both the crop yield and the amount of NO3 leached. Increasing irrigation uniformity increased crop yields but had variable effects on the amount of NO3 leached. A winter forage crop following a summer corn crop effectively reduced the leaching of residual soil N following the corn crop.

Introduction

Livestock and dairy production around the world is progressively moving toward congregating a large number of animals into small land areas. For example, dairies in California have a total herd size of 1.5 million cows. In 1999 the average size of California's 2200 dairy farms was over 650 milk cows, not including dry stocks, heifers, and calves (CDFA, 2000).

The feed rations for animals in the confined animal operations are formulated to maximize production. As a result, the large amount of nitrogen-rich wastes produced by the animals must be properly managed to avoid environmental degradation.

In the San Joaquin Valley of California, manure deposited on paved surfaces in dairies is removed with a flushing system. After separating out solids, the liquid manure is typically stored in ponds (lagoons) and eventually applied to adjacent cropland. Liquid manure applied to crop land serves as a fertilizer nutrient source for crops and may become a potential source of nitrate (NO3) groundwater degradation if the land applications are not properly managed.

Forage crops are capable of removing large quantities of N from the soil. Results of field investigations on the application of dairy effluent to year-round forage crops have been reported by Woodard et al. (2002), Hubbard et al. (1987), Vellidis et al. (1993), and Newton et al. (1995). The general findings were that the amount of N removal by the crop and the NO3-N in the soil water below the root zone tended to increase with increasing loading rates of N.

Nitrogen is present in the liquid manure in organic N (ON) and NH4+ forms. The latter is immediately available for crops but the ON must be mineralized before it is available for plant uptake. ON and NH4+ are not very mobile in soil, however, NH4+ can be nitrified to NO3 in days to weeks which is freely transported through the soil by flowing water. Proper management of liquid manure to maximize the fertilizer value and minimize water quality degradation requires knowledge of the complex dynamic interactions described above.

Dairies may employ different strategies in applying the liquid manures on cropland that entail different N inputs and timing of the applications. When different approaches of manure applications are adopted, it is difficult to project the outcomes in terms of crop yields and nitrate leaching due to the dynamic and interactive processes involving the reactions of applied N, irrigation, and plant growth. The temporal accounting of these coupled N reactions can be accomplished by utilizing a computer model such as the ENVIRO-GRO model (Pang and Letey, 1998). The model allows the simulation of various dairy liquid waste management options on water and nitrate distribution in the soil profile as a function of time, the amount of deep percolation, the amount of leached nitrate, and crop yield relative to that of a non-stressed crop.

The main features of the model are as follows: The one-dimensional Richards equation, which describes transient water flow through soil, is combined with a plant water uptake function. The water uptake function is based on the potential evapotranspiration (Tp) and the matric and osmotic head potentials of the soil water. The convection-dispersion equation is used to describe chemical flow. The model allows additional water and/or N uptake from zones in the root system where they are adequate to compensate for deficiency in other sections of the root zone. Since potential water and N uptakes are related to plant growth, a feedback mechanism is programmed so that reduced growth results in reduced potential water and N uptakes.

The goal of the research reported here was to use the ENVIRO-GRO model to simulate how the amount of applied N, timing of N applications, ON mineralization rates, chemical form of N applied, and irrigation uniformity affected (1) yields of corn (Zea mays) in summer and a forage grass in the winter in a Mediterranean climate and (2) the amount of NO3 leached below the root zone. The results can be used to guide the selection of management options to achieve desired goals.

Section snippets

Simulated farm management system

The cropping system typically used by dairy farmers in the San Joaquin Valley of California consists of planting silage corn in the spring and harvesting it in the fall, followed by a forage crop that is planted in November and harvested in April. In the simulations, we matched the irrigation and N applications with the requirements for crop growth. Dairy lagoon water was used as the only N source for the crops. Simulated irrigation was applied every 15 days with a mixture of lagoon water and

Organic nitrogen mineralization

Mineralization of N can be described using the first-order decay equation:Nmin=A0[1exp(λt)]where Nmin is the amount of mineralized N, A0 is the total amount of N in the organic material, t is time, and λ is the N mineralization coefficient.

Nitrogen mineralization is dependent on temperature (Frederick, 1950, Campbell et al., 1971). Stanford et al. (1973) estimated the rate constant at different temperatures. The relationship between mineralization rate and temperature is commonly described as

Irrigation uniformity

The simulated results are for the condition that the irrigation and nitrogen applications were uniform across the field; however, this condition is rare in an agricultural operation. The approach proposed by Letey et al. (1984) was used to determine the impact of non-uniform irrigation on the results. Because nitrogen was applied with the water in our case, zones receiving more water also received more nitrogen.

For any point “a” (finite size but small enough to be considered uniform),

Simulated variables

Field-average water application equal to 1.15Tp for the 15-day period since the last irrigation was used in all simulations. During the period between crops, it was assumed that there was no evaporation from the field. The potential water loss between the time of the last irrigation and the harvest of the crop was applied as an irrigation at the beginning of the next crop season.

The several variables combinations which were simulated are summarized in Table 1 for uniform irrigation and Table 2

Input data for the model

The simulations were conducted for a soil bulk density of 1.40 g cm−3 and a saturated water content of 0.48 cm3 cm−3. The saturated hydraulic conductivity was chosen at 2.0 cm h−1. The parameters used in the Hutson and Cass (1987) hydraulic function were as follows: water content at the inflection point (θi) was 0.48 cm3 cm−3; the matric potential at the inflection point (hi) was −0.0028 MPa; the air entry matric potential (a) was −0.0027 MPa; and exponent (b) of the equation relating matric potential to

Results

The organic N mineralization rate is plotted as a function of day of year for 5 years in Fig. 2 when Np = 1.4, summer half-life is 280 days, and the N was applied with each irrigation. Note that steady state values are reached after about 5 years. The temperol rate of mineralization does not coincide with the temperol rate of N uptake (Fig. 1). Therefore, the timing of leaching events will significantly affect the results. Large leaching rates at the initial and final stages of corn growth would

Conclusions

One major conclusion from this study is that when applying ON ultimately steady state conditions are achieved, whereby an equivalent amount of nitrogen applied in the organic form will be mineralized during a year. Steady state conditions are achieved more rapidly for materials with higher mineralization rates. This finding also underlines the importance that the results from short-term field experiments must be interpreted with caution. The experimental results will be very dependent upon the

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