C. L. Douglas

Because soil is a limited resource, agricultural production is dependent on improving soil quality. Improved soil quality also has an impact on water use, as high quality soil more effectively collects and stores water, reducing the need for irrigation. Intensive use of soil throughout history has led to depletion in soil quality, leading in turn to reduced yields because of the consequent reduced organic matter. Recognizing the lessons of history, scientists at research stations such as Rothamstead in England; Pendleton, Oregon; Champaign, Illinois; and Columbia, Missouri, began long-term studies on the effects of crop rotation, crop fertilization, manure additions, and residue management on the productivity and organic matter of cropped soils. In general, it was found that soil cultivation caused a decline in organic carbon content (which constitutes about half of the organic matter), or at best stabilized organic matter, even with heavy manure treatment, as long as conventional tillage continued. In the 1960s and 197Os, many investigators noted that tillage made soils more erodible, and that crop residues left on the surface were highly effective in reducing erosion. The introduction of more and …

Abstract Understanding microbial dynamics is important in the development of new management strategies to reverse declining organic‐matter content and fertility of agricultural soils. To determine the effects of crop rotation, crop residue management, and N fertilization, we measured changes in microbial biomass C and N and populations of several soil microbial groups in long‐term (58‐yr) plots under different winter wheat ( Triticum aestivum L.) crop rotations. Wheat‐fallow treatments included: wheat straw incorporated (5 Mg ha −1 ), no N fertilization; wheat straw incorporated, 90 kg N ha −1 ; wheat straw fall burned, no N fertilization; and wheat straw incorporated, 11 Mg barnyard manure ha −1 . Annual‐crop treatments were: continuous wheat, straw incorporated, 90 kg N ha −1 ; wheat‐pea ( Pisum sativum L.) rotation (25 yr), wheat and pea straw incorporated, 90 kg N ha −1 applied to wheat; and continuous grass pasture. Total soil and microbial biomass C and N contents were significantly greater in annual‐crop than wheat‐fallow rotations, except when manure was applied. Microbial biomass C in annual‐crop and wheat‐fallow rotations averaged 50 and 25%, respectively, of that in grass pasture. Residue management significantly influenced the level of microbial biomass C; for example, burning residues reduced microbial biomass to 57% of that in plots receiving barnyard manure. Microbial C represented 4.3, 2.8, and 2.2% and microbial N 5.3, 4.9, and 3.3% of total soil C and N under grass pasture, annual cropping, and wheat‐fallow, respectively. Both microbial counts and microbial biomass were higher in early spring than other seasons. Annual cropping significantly reduced declines in soil organic matter and soil microbial biomass.

Abstract Rhizobacteria have the potential to suppress plant growth. We evaluated the effect of native pseudomonads on downy brome ( Bromus tectorum L.), a troublesome weed in small‐grain‐producing lands. Pseudomonas spp. were isolated from winter wheat ( Triticum aestivum L.) and downy brome roots and tested to determine their potential as biological control agents for this weed. Pseudomonads were screened in agar and soil for inhibition of downy brome root growth and lack of inhibition of winter wheat root growth. Of more than 1000 isolates tested, 81 inhibited downy brome and not winter wheat in the agar seedling bioassay. Six isolates consistently inhibited downy brome growth and not winter wheat in soil contained in pots in the growth chamber. In nursery field trials in which downy brome was planted in rows and adequately fertilized, some of the bacterial isolates reduced downy brome populations up to 30% and shoot dry weight up to 42%. Field studies were also conducted at three sites in eastern Washington in which brome‐inhibitory bacteria were applied to wheat fields infested with natural populations of downy brome. Of the three isolates studied in the field, two reduced plant populations and aboveground growth of downy brome up to 31 and 53%, respectively, compared with noninoculated controls. At two of the three locations, winter wheat yields were increased 18 to 35% because of the suppression of downy brome growth. Brome‐suppressive bacteria isolated from the rhizoplane of winter wheat and downy brome can be used as biological control agents for downy brome.

Abstract Field‐measured characteristics of cereal residue decomposition under semiarid dryland agriculture are needed for systematic management of residue in reduced tillage systems. Straw placement, loading rate, and nutrients are all important characteristics. Straw composition and placement, at a moderate loading rate, were evaluated for their effects on decomposition. Wheat straw with three different N and S contents was contained in fiberglass cloth bags which were placed above, on, and below the soil surface to simulate, respectively, standing stubble, straw matted on the surface, and straw plowed under. Weight loss and changes in N and S content were measured during a 26‐month period in a winter wheat ( Triticum aestivum L.)‐pea ( Pisum sativum L.) rotation. After 26 months exposure residue losses were 25, 31, and 85% for placements above, on, and in the soil, respectively. Above‐surface and on‐surface straw showed a nearly constant decomposition rate with little response to weather variables; decomposition rate of buried straws responded to both soil moisture and temperature. Mineralization or immobilization of N and S were sensitive to placement and initial nutrient content of the straws. Net N mineralization in buried straw varied from 17 to 2 kg N/ha as straw composition varied from 0.78 to 0.20% N. When placed above or on the surface net N mineralizations ranged from 6 to −4 kg N/ha as straw varied from 0.78 to 0.20% N. Net S mineralization paralleled N mineralization, however, magnitudes were smaller. First‐order rate constants for weight loss successfully characterized the placement and straw composition effects.

ABSTRACT: Soil organic carbon (SOC) makes up about two-thirds of the C pool in the terrestrial biosphere; annual C deposition and decomposition to release carbon dioxide (CO2 into the atmospheric constitutes about 4% of this SOC pool. Cropland is an important, highly managed component of the biosphere. Among the many managed components of cropland are the production of crop residue, use of tillage systems to control crop residue placement/disturbance, and residue decomposition. An accumulation of SOC is a C sink (a net gain from atmospheric CO2) whereas a net loss of SOC is a C source to atmospheric CO2. A simple three components model was developed to determine whether or not conservation tillage systems were changing cropland from a C source to a C sink. Grain/oil seed yields and harvest indices have indicated a steadily increasing supply of crop residue since 1940, and long term field experiments indicate SOC storage in no-tillage > non moldboard tillage > moldboard tillage systems. According to adoption surveys, moldboard tillage dominated until about 1970, but non moldboard systems are now used nationally on at least 92% of planted wheat, corn, soybean, and sorghum. Consequently, since about 1980, cropland agriculture has become a C sink. Moldboard plow systems had prevented a C sink response to increases in crop residue production that had occurred between 1940 and 1970. The model has not only facilitated a qualitative conclusion about SOC but it has also been used to project production, as well as soil and water conservation benefits, when a C credit or payment to farmers is associated with the C sink in cropland agriculture.