Our results show that nutrient loading in transported sediments is highest at the beginning of the winter rainy season for N03, NH4, and labile P. We did not collect samples from sediment fans deposited after the initial rain events of the season . But based on these numbers from slightly later in the season, we hypothesize that nutrient loading is highest during the first rain events after planting. There are several reasons for this. One is the presence of fertilizer nutrients in freshly cultivated and fumigated soils in the absence of plants with big enough root systems to take up those materials. Establishment of the plants and increased nutrient uptake later in the winter when the later samples were collected may account for decreased levels ofN and P in sediments at those times. Enhanced levels ofN and P in early winter run-off may also originate from a soil reservoir of fertilizer-applied nutrients applied in years past which are made available by the turning under of the crop and the intense cultivation of the soil prior to fumigation and planting. Lower levels of sediment Nand P later in the rainy season may also reflect a net movement of these nutrients into the wetlands. The marshes harbored a restricted fauna, relative to the fauna found in the adjoining slough . The South Marsh showed the lowest abundance and diversity, with almost no living macrofauna collected from the benthos. In addition,wholesale planter containers the sediment in the central portion of this marsh was completely anoxic. The Central Marsh likewise showed few infaunal invertebrates. This may also be a reflection of the changeable water quality and anoxic sediments. The North Marsh showed the greatest abundance and diversity c; .nfaunal invertebrates of the three Azevedo marshes. This consistent with the greater flushing of this marsh with tidal water.

The distribution of infaunal invertebrates within the three marshes tracks the relative degree of disturbance in each. The least flushed, most disturbed marsh showed no infauna while the largest, best flushed and least disturbed marsh showed the greatest diversity. All of these marshes exhibited a restricted fauna relative to the control pond on the Reserve. Future sampling will examine more of the seasonal changes associated with these ponds and will work to link land use practices with the health of the infauna. The 1 acre South Marsh is the smallest of the three and is cut off from tidal exchange . It has experienced the greatest degree of filing from the adjoining agricultural operations and is surrounded by the smallest buffer of salt marsh. This pond had a growth of Ruppia maritima in July, 1992 that was heavily encrusted with consolidated sediment, perhaps cemented by a bacteria or protozoan. This mat of Ruppia formed a false bottom in the pond above which were pupae of the brine flies and corixid beetles. The underlying sediment was completely anoxic and no living infauna were retrieved from the cores in 1992. The summer 1993 samples contained one oligochaete and several corixid beetles. Again, the sediments were completely anoxic, although the Ruppia canopy was not present. The Central Marsh is 4.1 acres in extent and is intermediate in size and disturbance . The central pond is blocked from tidal action by high culverts and by a low berm across the mouth of the marsh . This marsh also receives direct freshwater input during the rainy season from a culvert draining an agricultural pond above the road. Sediments in this pond were oxidized on the surface, but anoxic a few millimeters below the surface. The infauna reflected these difficult conditions and few species were recovered from either station on either sampling date. The North Marsh is the largest of the three at 10.1 acres .

It is connected to the main channel of the slough through two culverts, one at either end of the marsh. This marsh receives the most tidal flushing, though the central portions of the marsh are not well flushed. In all cases, the North Marsh showed the greatest number and diversity of infaunaI species of the three Azevedo marshes. Samples taken in July 1993 showed an absence of many of the soft-bodied species collected in October of 1992. In addition, the presence of a podocopid ostracod in July was notable. Many dead ostracods and shells were recovered from these samples. In some areas, evidence of anoxic waters and sediments was observed, and dead ostracod shells found. The control pond on the National Estuarine Research Reserve served as a contrast to the Azevedo marshes. It is more fully flushed, has never been cultivated, and is undisturbed relative to the Azevedo marshes. Infaunal samples showed greater species diversity in this pond relative to the Azevedo marshes. The invertebrate community in the control pond was more similar to that found in the main channel of the slough than the Azevedo marsh community. Methyl bromide is a soil fumigant of environmental concern because of its high potential to deplete stratospheric ozone . A treaty signed by 160 nations of the United Nations Environment Program regulates the stepwise decrease of MeBr consumption to a complete phase-out by January 2005 for developed countries and by 2015 for developing countries . The stringent regulations limiting the use of MeBr prior to its complete phase-out stimulated the search for alternative fumigants because soil fumigation remains a central tool in strawberry production. For the past 45 years, preplant fumigation of agricultural soils with a combination of MeBr and chloropicrin has been a reliable and effective tool to control soilborne pathogens, nematodes and weeds in many vegetable, fruit, nuts and nursery crops worldwide. The irritant compound CP is added to the odorless MeBr as a warning agent to reduce the risk of accidents during soil fumigation and because of the synergistic biocidal effect of these two chemicals on soil pathogens . The elimination of MeBr could severely impact growers and farmers in the United States and the Mediterranean region.

In continuous strawberry production systems, the soil may host many deleterious nematodes and pathogens such as Phytophtora cactorum, P. fragariae, Verticillium dahliae and Colletotrichum acutatum. In California, where 80% of US strawberries are grown, MeBr + CP combinations effectively control wilt disease , thus playing a crucial role in commercial strawberry production. Currently, there are several available alternatives to MeBr, including an emulsifiable concentrate of CP and 1,3-dichloropropene . Applied alone, CP has high biocidal activity against fungal pathogens but is not as effective as MeBr against weeds and nematodes. Another viable alternative is 1,3-dichloropropene , which is an effective nematicide but has relatively low activity against fungi and weeds . To broaden its biocidal activity, 1,3-D can be combined with chloropicrin as found in various combinations such as InLine . In addition, several experimental chemical alternatives are being studied for their efficacies against pathogens and pests. Iodomethane can be as effective as MeBr,plastic pot manufacturers and it is not as likely to deplete ozone because Midas is photolyzed before it reaches the stratosphere . The dilution of Midas with CP can decrease costs for this fumigant and increase efficacy due to synergy with CP . Another experimental chemical alternative is propargyl bromide , which was developed during the 1960s. Although, PrBr demonstrated potential as a viable MeBr replacement it was never registered due to its highly explosive character . With the development of a stabilized formulation of PrBr, research interest in this compound as a soil fumigant has increased recently. Studies have been conducted to determine the biological degradation of various fumigants in soil and their efficacies against soilborne pests and weeds relative to MeBr + CP combinations . Fumigants are among the pesticides with notable effects on soil microorganisms because of their broad biocidal activity . The high biocidal activity of fumigants may cause a “biological vacuum” and increase pathogen re-colonization. Kandeler et al. suggested that the composition of the microbial community strongly affects the potential of a soil for enzyme-mediated substrate catalysis. Consequently, changes in microbial diversity in fumigated soils may also reduce microbial functionality. Enzyme activities can be used as an index of microbial functional diversity , although accumulated enzymes may contribute considerably to the overall enzyme activity of a soil. A semi-quantitative method to determine enzyme protein contents in soil based on the specific activities of reference enzymes and enzyme activity values of soils was reported by Klose and Tabatabai in order to prove whether there is a direct correlation between the activity of any enzyme and its protein concentration in soil. This approach is based on the assumption that the compositions of the reference enzymes are similar to those in soils. Protein concentrations were suggested to serve as a suitable measure to quantify the effects of environmental changes, for example after application of pesticides, on soil biological properties . The understanding of the impacts of pesticide fumigants on key biochemical reactions involved in organic matter degradation and soil nutrient dynamics is important in order to evaluate the ecological significance of fumigation on the soil system. The toxicity of fumigants is related to their interference with respiratory enzymes, including pyruvate dehydrogenase, their ability to chelate metal cations such as Cu, the inhibition by the unchelated ion, and toxic degradation products such as methyl isothiocyanate.

MeBr can be degraded in soils by the following three pathways : a) chemical hydrolysis to form methanol and bromide, b) methylation to soil organic matter and release of bromide ion, and c) microbial oxidation to form formaldehyde and bromide ion. Biological hydrolysis and other microbial processes involving enzymatic processes are also likely to contribute to the degradation of MeBr in soil . Microbial respiration, nitrification potential, and dehydrogenase and arylsulfatase activities were inhibited by MeBr + CP and the alternatives PrBr, InLine, Midas and CPEC one week after soil fumigation . After 30 weeks, there was no difference in microbial biomass and activities between the treatments studied, with the exception of lower acid phosphatase and arylsulfatase activities in fumigated soils. These results indicate that there are short- and long-term differences in the response of various microbial and enzymatic processes to MeBr + CP and alternative fumigants and thus, of the various functions of the soil biota in ecosystems. A limitation of this study is that it was conducted for a maximum of 37 weeks; it remains unknown if MeBr + CP and alternative fumigants have longer-term impacts on soil biochemical processes under field conditions after multiple applications. The objective of this study was to evaluate the effect of repeated soil fumigation with MeBr + CP and two registered and two non-registered alternative fumigants on microbial biomass and respiration, the activities of dehydrogenase, acid phosphatase, β-glucosidase and arylsulfatase, and enzyme protein concentrations in soils. Furthermore, the effect of these fumigants was evaluated on dry proteins containing β-glucosidase, acid phosphatase and arylsulfatase in the absence of immobilizing or protecting constituents of soil . The selected alternative fumigants represent the actual formulations that likely will be used by growers for strawberry production. Dehydrogenase activities were selected because they reflect the total oxidative activities of soil microorganisms and are important in oxidizing soil organic matter. Acid phosphatase catalyzes the hydrolysis of a variety of organic phosphomonoesters and is therefore important in soil organic P mineralization and plant nutrition. The enzyme β-glucosidase catalyzes the hydrolysis of cellobiose, and thus plays a major role in the initial phases of the decomposition of organic C compounds. Arylsulfatase is believed to be partly responsible for S cycling in soils as it participates in the process whereby organic sulfate esters are mineralized and made available for plants. The first aim of the present study was to test whether soil fumigation with these four potential pesticides will alter important soil functions that, in turn, will affect the long-term productivity of agricultural soils. The second aim of this study was to evaluate the effects of soil fumigation on the activities of enzyme proteins, which may be present in the soil as free enzymes and not protected by clay-humus complexes. Free enzymes are likely to be more sensitive to environmental factors as intracellular or adsorbed enzymes, which are protected by the cell envelope or by clay-humic complexes.