Runoff, sediment, and nutrient export from manured riparian pasture as affected by simulated rain and ground cover
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Date
2004-09-02
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Abstract
The impact of livestock pastures on sediment and nutrient export to surface waters in North Carolina is not well documented. The objective of this work was to determine the effect of ground cover on sediment and nutrient export from pastured riparian areas. In one experiment, plots 0.75 m by 2.0 m were established on 10% and 20% slopes of existing stands of mixed tall fescue (Festuca arundinacea Schreb.) / dallisgrass (Paspalum dilatatum Poir.), with stands modified to represent a range of ground cover levels by establishing 100% bare soil area with 0% ground cover (bare ground), 55% bare soil area with 45% ground cover (low cover), 30% bare soil area with 70% ground cover (medium cover), and not altering full vegetative cover plots (high cover). The bare ground treatment was also compacted to simulate a cattle heavy use area. Measured forage canopy cover on vegetated plots was generally higher than the level of ground cover established at the soil surface. At 45% ground cover, canopy cover was 63 to 78%, at 70% ground cover canopy cover was 76 to 83%, and at full ground cover canopy cover was 83 to 98%. In a second experiment, plots 0.75 m by 2.0 m were established on stands of existing wetland vegetation that consisted of pickerelweed (Pontederia cordata L.), showy goldenrod (Solidago erecta (Pursh) MacM.), Japanese honeysuckle (Lonicera japonica Thunb.), arrowleaf tearthumb (Polygonum sagittatum L.), and leathery rush (Juncus coriaceus Mackenzie), with stands modified to establish either no ground cover (bare ground) or to allow full cover. Rainfall simulators were used to evaluate runoff, total suspended sediment (TSS), total Kjeldahl nitrogen (TKN), nitrate-N (NO3-N), ammonium-N (NH4-N), total Kjeldahl phosphorus (TKP), and dissolved reactive P (DRP) export from plots with applied cattle (Bos spp.) feces and urine at a rate representing stocking of ~1.4 cows ha-1 yr-1. Rainfall simulation events were conducted in April 2003 to evaluate baseline runoff conditions. Feces and urine were applied in May and Sept 2003 and immediately followed with rainfall simulation events. Additional rainfall simulations without additional application of feces and urine occurred in June and Oct 2003.
In the first experiment involving mixed tall fescue / dallisgrass, mean runoff volume from bare ground was generally greater than from low, medium, and high levels of cover, which had similar mean runoff volume. Export of TSS was greater from 20% than 10% slope on bare ground and low cover, but when cover was medium or high, TSS export was similar at both slopes. Export of DRP was elevated after application of feces and urine to plots, generally with the greatest export in Sept. However, there was not a consistent response of DRP export to cover at each rain event. Mean TKP export was similar at low, medium, and high cover levels over all levels of slope and rain event, suggesting that low cover may be sufficient to preventing excessive TKP export. Mean NO3-N export was greatest from bare ground and few differences were observed at low, medium, and high cover levels. Mean NO3-N export was also greatest during Oct, whereas there were no differences between May, June, and Sept rain events. Mean NH4-N export was elevated (~1.37 kg N ha-1) in months when feces and urine were applied and minimal (< 0.05 kg N ha-1) in all other months. Mean export of both TKN and TN was greatest at bare ground and did not differ at low, medium, and high cover levels at each rainfall event except June, where mean TKN and TN export at bare ground did not differ from high cover.
In the second experiment on existing wetland vegetation, mean runoff volume was generally greater from bare ground than full cover, and from the wetland plots compared to upland mixed tall fescue / dallisgrass plots on a similar 10% slope. Plots with full cover were remarkably effective at minimizing mass export of TSS during all simulated rain events. There was a greater mean export of DRP from plots with full cover compared to plots at bare ground, though mean export at both levels of cover was < 0.20 kg P ha-1. Unlike DRP, mean TKP export was much greater from bare ground than full cover, which is not surprising given the large amount of TSS exported from bare ground plots. Greater mean NO3-N export was observed from the bare ground treatment than from full cover, as well as one month following application of feces and urine. Conversely, rain events that included feces and urine application had a far greater export of NH4-N, TN, and TKN than did rain events without feces and urine application.
Results indicate that livestock heavy use areas in riparian zones may export substantial sediment, N, and P, but when cover is maintained sediment and nutrient loss from cattle feces and urine may be minimal. Cover and time of rainfall following grazing are important determinants of sediment, N, and P export to surface waters.
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Keywords
nitrogen, pasture, phosphorus, riparian grazing, runoff
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Degree
MS
Discipline
Crop Science
