To protect water quality, California regional water quality control boards have adopted different measures to regulate pollutants in water from agricultural operations, including implementation of best management practices . Typical pollutants in areas with exceedances of total maximum daily loads include nitrogen, sediment, phosphorus and pesticides, such as chlorpyrifos. Many surface water TMDL exceedances occur during the rainy season, indicating the need for practices that address stormwater runoff. In some areas, management practices that reduce rain-induced leaching of soluble pollutants into groundwater may also be needed.In this project we compared treatment efficacies and costs of untreated tunnel post rows with rows treated under four different practices in plastic-covered raspberry operations at Somis and Santa Maria , California. Both sites were on moderate slopes , but the beds were planted on the contour to reduce runoff and were on a 1% slope at both sites. The post row treatments were selected based on previous work and potential feasibility for caneberry operations. Each treatment was applied to 6-foot-by-300-foot post rows in an experiment with randomized complete block design with three replications at both sites. Site conditions are described in table 1. The project focused on the rainy seasons of 2016–2017 and 2017–2018. The four treatments were as follows: A barley cover crop was seeded in 2016 at 500 pounds per acre with a seed spreader, planting blueberries in pots lightly raked into the soil and established with sprinkler irrigation used for delivering overhead water to newly planted raspberry roots .
At both locations we reseeded barley at the same rate during the second rainy period of the project to increase cover crop density in areas lacking ground cover. Weed block fabric is commonly used in organic and hydroponic production systems. Fabric was unrolled and pinned by hand to cover the post-row surface between raspberry beds prior to post installation. The fabric remained in place during the experiment and was unpinned and rolled up at the end of the project for potential reuse. Yard waste mulch from local suppliers was delivered to the project sites. Mulch was a woody < 2-inch screened material with < 20% fine components. Different mulch sources at the two sites were used because the distance between sites and volume requirements for each site were prohibitively large to source from a single supplier. Mulch was delivered by tractor to post rows, where it was spread with rakes to cover the entire post row with a 2- to 3-inch thick layer. At both locations mulch was applied once prior to post installation and persisted throughout the trial period. Polyacrylamide , a nontoxic soil-binding polymer, was applied prior to rain events at a rate of 2 pounds per acre. In 2016–2017, PAM was mixed with water and applied with a backpack sprayer, but due to plugging of nozzles we dispersed dry PAM to post rows instead in 2017–2018 and observed similar efficacy and increased ease of application.In the 2016–2017 season, we collected runoff samples by hand within 30 min from the beginning of the runoff generation, approximately 25 feet away from the ends of each of the treatment post rows .
About 250 milliliters of runoff water in each sample were brought from field sites to the UC Cooperative Extension Ventura County lab and immediately tested for turbidity using a turbidimeter , acidified with sulfuric acid to reach pH 3 and either shipped immediately to the ANR analytical lab at UC Riverside or stored at 4°C until shipment. Levels of nitrogen forms and total nitrogren and phosphorus were determined using aDiscrete Analyzer AQ2 . In 2017–2018, we collected grab samples as described above. We also collected runoff in 5-gallon buckets installed at 25 feet from the end of post rows to intercept first flush of runoff at soil surface level. Additionally, we installed suction lysimeters about 30 feet away from the ends of the post rows at 8-inch depth at Santa Maria and 8- and 24-inch depths at Somis and collected leachate after rains. In 2017–2018 we also collected sediment from the buckets after runoff occurred, and the sediment samples were dried and weighed at the UCCE Ventura County lab. In April 2018, we took soil samples that were analyzed for soil moisture, nitrate nitrogen and phosphorus content.Weed numbers were determined by counting all germinated weeds in each 1,800–square foot plot at each site on three dates. Predominant weed species at Somis were little mallow and annual sowthistle , and horse weed and annual bluegrass at Santa Maria. Additionally, in April 2018 at Somis we counted the numbers of volunteer raspberry shoots in all plots. Runoff, weed and cane data were analyzed using the GLM Procedure in SAS with the overall error rate controlled by Tukey-Kramer adjustment.We calculated the costs of each treatment for the 1,800– square foot experiment plot and then extrapolated the costs into a per acre basis for one tunnel use period.
A tunnel use period covers a 3-year production cycle of raspberry from establishment until termination. Costs of treatments included materials, labor and equipment when applicable. We also adjusted the treatment’s costs if it provided weed control benefit. In addition, some treatments can serve for more than one tunnel use period. Therefore, we distributed the costs accordingly.Not all treatments had runoff during light rains. Barley cover crop and yard waste mulch likely interfered with low flows and aided water retention in post rows. We observed slower flows and greater puddling in post rows with barley or mulch than in other treatments or untreated soil . Soil sampled 3 days after rain in March 2018 at Somis had 8% to 12% greater moisture content at both sampling depths under mulch compared with other treatments . Mulch also conserved more soil moisture than fabric at Santa Maria .Combined nitrite and nitrate levels in runoff samples ranged from 0.29 to 6.48 milligrams per liter over two seasons of sampling. This variability is due to the intensity and frequency of the rains during this period, which also affected the accumulated fertigated nitrogen that occurred between rain events. Fabric and PAM did not reduce nitrate or nitrite in runoff compared with untreated soil at any of the sampling dates at both locations and sampling seasons , while mulch was equally ineffective in 2016–2017 in reducing NOx in runoff at both locations. During one out of five runoff events in 2016–2017, barley reduced NOx levels in runoff by 48% compared with untreated soil, but not significantly during other rain events of that season . During two out of five runoff events at Somis in 2017–2018, barley reduced NOx levels in runoff by 71% and 82% and mulch reduced them by 67% and 91% compared with untreated soil, but reductions were not significant at other sampling events. At Santa Maria, none of the treatments had significant impact on NOx in runoff when compared with untreated soil . All treatments at Somis were effective in reducing ammonium in runoff in 2016–2017 compared with untreated soil , but only barley was effective in 2017–2018. The overall greater average levels of ammonium in 2017–2018 were likely due to use of passive samplers that intercepted the first flush of runoff, which may have had a greater concentration of pollutants than runoff collected later . Ammonium is typically carried on sediments, so lower ammonium would indicate less sediment movement. This suggests that barley cover crop and yard waste mulch can reduce both the concentration of dissolved ammonium nitrogen in runoff and the volume of runoff, leading to potential reductions in nitrogen losses to the environment compared with untreated soil.Soil under barley and mulch had significantly less nitrate nitrogen compared with other treatments in March 2018 at Somis . At Santa Maria, all treatments except for mulch had 25% to 81% less nitrate nitrogen than that of untreated soil, planting blueberries in a pot although mulch was also similar to all other treatments. Mulch deterioration might have reduced its efficacy at Santa Maria. At Santa Maria, nitrate nitrogen levels in leachate collected at 8-inch depth on all sampling dates ranged from 12 to 27 parts per million in PAM and untreated plots, which was 52% to 80% greater than those in other treatments . At Somis a similar trend was observed: nitrate nitrogen levels in leachate under PAM and untreated soil were 7 to 22 ppm, which was 80% to 90% greater than those under barley or mulch. Leachate nitrate concentrations under fabric were not different from those in untreated soil . These results suggest that barley and mulch can reduce nitrate nitrogen in soil and leachate.
Mulch and cover crop act as a barrier to runoff water with dissolved nitrogen and sediment and may retain nitrogen to be used for cover crop growth and for residue and mulch decomposition.Turbidity in first flush of runoff was reduced 5- to 10-fold by all treatments compared with untreated soil at both locations in 2018 . These results were similar to turbidity in grab samples taken in 2017 and 2018 , which suggests that all treatments were effective in reducing waterborne sediments on site. Additionally, 75% to 97% less sediment was collected from passive samplers in all treated post rows compared with those in untreated soil, as shown for March 10, 2018 . Relatively high sediment load in fabric treatment resulted from deposits of soil on top of the fabric during removal of plastic from raspberry beds. Similar to the March 10 rain event, we observed significantly lower sediment levels after other rains in all treated post rows compared with untreated rows . We also observed fewer erosion channels in treated post rows compared with untreated plots at both sites during the trial. Besides the agronomic benefits, retaining soil in the field is also a good pesticide management practice because soil-adsorbed pesticides will stay in the field and not end up in receiving bodies of water. In a previous study, Mangiafico et al. showed that concentrations of the harmful insecticide chlorpyrifos in runoff were linearly related to sample turbidity. This suggests that retaining waterborne sediments on-site is an effective method for mitigating runoff of this pesticide. Preventing soil movement with these post row treatments may also reduce the costs of sediment removal from receiving waterways and associated environmental impacts . Phosphorus levels in the first flush of runoff samples were reduced by 24% to 85% in all treatments compared with untreated soil at Somis in 2018, except for PAM on Feb. 27, 2018 . Lack of efficacy of PAM on that date may have resulted from deterioration of the PAM seal due to soil disturbance after PAM application and before runoff sample collection. At Somis in 2016–2017 and Santa Maria in 2018, we observed a similar reduction in phosphorus by all post row treatments compared with untreated soil . Since phosphorus is normally adsorbed to soil particles , reduction in turbidity and phosphorus in runoff samples from treated post rows followed a similar trend. Reducing losses of phosphorus from production fields may help prevent eutrophication in receiving waterways when this microelement is limiting for algal growth .Since tunnel post rows receive water and retain soil moisture, conditions are favorable for weed growth. At both locations weed barrier fabric provided nearly complete weed control with only occasional weed germination in areas where soil was deposited on the top of the fabric. Application of PAM did not provide control, and weed densities in PAM-treated rows were similar to those in untreated plots. Yard waste mulch provided 81% to 90% weed control at Somis but did not control weeds in two out of three evaluation dates at Santa Maria . Mulch at Santa Maria was much finer compared with the one at Somis, and likely decomposed more rapidly, allowing weed growth. Barley cover crop provided 86% and 42% weed control on two evaluation dates at Somis, but after barley was reseeded, high germination of little mallow occurred . Incorporation of barley during reseeding likely disturbed hard-coated weed seeds sufficiently to break dormancy; however, mallow was controlled before seed production when barley was mowed in spring. Barley cover cropat Santa Maria provided 87% and 43% weed control at two out of three evaluation dates.