Agricultural crop production is a biological process; consequently, plant growth, and many parameters affecting plant growth, are controlled by both the biological and physical environment of the soil. The biological component includes the growing crop and other living organisms in the soil environment, especially the soil microorganisms. All living organisms respond to their physical environment in accord with the principals of ecology, i.e., the growth and activity of each organisms depend upon the degree to which its environmental requirements are met.

Since the growing crop is a biological organism, through choice of farming practices we seek to optimize environmental conditions for the growing crop. However, environmental factors influencing crop growth, such as nutrient availability, root aeration, certain hydraulic characteristics, and other parameters, are often altered and controlled in a major way by microbiological activity within the soil. Microbiological activity, in turn, depends upon soil temperature, water, aeration, and available C or substrate in the soil. Thus, there is considerable feedback in that the physical and biological environments within the soil are both caused and affected by biological activity.

With this sequence of events in mind the agricultural ecosystem can be viewed as diagrammed in Figure 1. Through choice of tillage and other management practices applied to certain soil in a certain climate, the farmers regulates, to a large extent, the physical soil environment--especially aeration, water, temperature, and substrate (C sources) availability. According to the principles of microbial ecology, soil organisms respond to this environment in terms of their proliferation and level of activity. Thus, if management practices result in reduced aeration, anaerobic processes will be enhanced and aerobic processes suppressed. As a result of these activities, nutrient availability may be altered, soil aggregation affected, porosity changed, root disease vectors influenced, and other parameters affected. These consequences of microbial activity can affect plant root growth, nutrient uptake, and, ultimately, crop yield. Thus, the farmer exercises control of this sequence of events primarily by his/her choice of tillage and other management practices employed.

Parameters of the soil physical environment of major concern are those that affect the water, temperature, and aeration regimes experienced by soil organisms and plant roots. A number of studies have documented effects of tillage practices on several soil properties and have shown that leaving crop residues on the soil surface reduce evaporation rate and improve the conservation of soil water (Black, 1973; Campbell et al. 1976; Zingg and

Whitfield, 1957). Such practices usually reduce soil temperatures by several degrees. Soil bulk density is often greater with no tillage than with tillage. This fact, coupled with the greater water content of no-tilled and reduced-tilled soils, results in a larger percentage of the soil pores being filled with water. Under these conditions, oxygen diffusion may more frequently limit biological activity in soils managed with reduced or no tillage than with clean cultivation (Linn and Doran, 1984).

Data from tillage experiments conducted at Sidney, Nebraska, (Table1) illustrate the effects of tillage practices on several factors that influence the soil physical environment (Broder et al. 1984, Mielke et al. 1984). Although data presented represent only one sampling date, similar treatment effects were found at other sampling dates over a 4-year period. These data are typical of those reported in several other publications (Blevins et al. 1983; Fenster and Peterson, 1979; Unger and McCalla, 1 980). Compared to plowed soil, values of all parameters listed except air permeability were greater in the no-tilled soil. Because of its greater water content, air permeability of no-tilled soil was less than that of plowed soil. The effects of tillage treatment on soil physical properties usually tended to decrease with depth.

One can conclude from the data in Table 1 that no-tilled soils tend to be wetter than plowed soils; suggesting that pore continuity was destroyed by tillage. The greater water content and higher percent water-filled pores in no-tilled soil result in slower rates of air (oxygen) diffusion, so one would expect a less oxidative environment in no-tilled soil. In a semiarid environment such as western Nebraska, however, only during rainy periods would oxygen diffusion rate be suboptimum for aerobic activity.

Because of the greater water content and the insulating effect of surface residues, normally soil temperatures in reduced and no-tilled soils are 2 to 8 ° F cooler than for bare soil (Unger and McCalla, 1980). This trend was observed here in that mean weekly temperatures in the surface 6 inches were 3 to 7° F cooler for no-tilled than for plowed soil. Reduced soil temperatures would also decrease rate of oxidation of crop residues and soil organic matter. Thus, the soil physical environment for no till, compared to plowed soil, is characterized as being cooler and wetter, and would be less oxidative during much of the growing season.

As a consequence of differences in soil physical environment resulting from different tillage practices (Table 1), microbial habitat (ecological niches) existing at the myriad of microsites in the soil also differs between tillage treatments. Because of the warmer and drier environment in plowed soil, growth and activity of fungi are favored over those of bacteria. In surface soils, however, total numbers and biomass of microbes may be greater for no-tilled than for plowed soil because of the more nearly optimum soil water content and a concentration of carbon-rich crop residues at the surface of no-tilled soil. Thus, microorganisms near the surface of no-tilled soils would have available to them both more water and greater quantities of substrate (soluble C) than would be found in plowed soil. At lower depths, the reverse might be expected because plowing would cover surface residues to depths of 8 inches or greater, and soil water content would be greater than in surface soil.

In the experiments at Sidney, Nebraska, various assessments were made to determine the effect of soil environment (as affected by tillage) on microbial growth and activity (Table 2). As predicted above, populations of all classes of microorganisms were greater in the surface soil of no-tilled than in plowed fallow. Likewise, microbial biomass (total weight of all microorganisms) in this depth was greater for no till. Below the surface layer, however, populations and weight of all organisms except denitrifiers were less for the no-till soil. These are the kinds of differences one would expect because of the cooler and wetter environment, the enhanced availability of water, and the greater mass of crop residues at the surface of no-tilled soil.

Results similar to those shown in Table 2 have been published from a number of locations (Broder et al. 1984; Linn and Doran, 1984; Smith et al. 1980). In a study of seven tillage experiments from West Virginia to Oregon and including these experiments in western Nebraska, Doran (1980) obtained results very similar to those reported in Table 2. Populations of all aerobic organisms (fungi actinimycetes, bacteria, and both NH ⁺₄

and NO ^–₂ oxidizers) plus facilitative anaerobes in no-tilled compared to plowed soil increased from 14 to 57 % in the surface 3 inches, while denitrifiers increased several fold. Dehydrogenase activity in surface soil was 10 to 190% greater, and phosphatase activity was 10 to 50% greater for no-tilled than for plowed soil.

At several sites across the United States, Linn and Doran (1984) measured the amount of C0₂ and N₂0 that accumulated in the headspace above the soil surface in a closed container inserted 3 inches into field soils. Concentrations of C0₂ and N₂0 would be related to total microbial activity and activity of denitrifiers, respectively. They found that average C0₂ and N₂0 concentrations in the headspace were 2 to 3 times greater for no-tilled than for plowed soil (Table 3). They also found that N fertilization had little or no effect on C0₂ accumulations, but increased N₂0 accumulation by an order of magnitude for both tillage treatments. Thus, these results also suggest that no till greatly enhances microbial activity in surface soils, including that both aerobic and anaerobic organisms.

Altered soil microbiological activity resulting from changes in soil physical environment created by tillage altars transformations of N and other nutrients. These changes, in turn, affect nutrient availability to the crop (Figure 1). Generally, any change in one part of the ecosystem may affect several other biological and physical components. Thus, the effect of tillage on N availability may result from the collective effect of a number of factors affecting the N cycle simultaneously.

A useful diagram of the N cycle is presented in Figure 2. Microbial biomass is central to this cycle because most mobilization-immobilization reactions occurring in the soil are microbial mediated (Paul and Juma, 1981). The soil organic-N pool can be visualized as being composed of several compartments, varying in chemical composition and relative rate of mineralization. In Figure. 2, three such pools are visualized--one composed of N immobilized in microbial biomass and other readily labile N forms; the second composed of less readily hydrolizable N compounds (relatively labile) such as might be found in some crop roots and residues; and the third composed of soil humates and other resistant cyclic N compounds (stable 1 organic N). One can think of the first pool as containing readily labile N with a turnover rate of a growing season or even less; the second pool of slowly labile material with a turnover rate of years, even a decade; and the third pool of more stable material with turnover rates measured in decades or centuries.

As an example of the effects of tillage practices on soil N pools and availability, the quantity of N in several components of the N cycle at Sidney, Nebraska, is given in Table 4. Soil water content for no till was greater than that for other tillage methods (Table 1). Both total soil N and organic C were greater in the upper 3 inches of soil for no till than for other tillage methods. Tillage had little effect on these parameters at 3 to 6 inches. Again, similar results have been reported at other dates and at numerous other locations. Tillage had little effect on soil NH ⁺₄-N concentrations at either depth or on nitrate concentrations in the upper 3 inches. Below this depth, however, nitrates were lowest for no till. No till increased potentially mineralizable N in the upper 3 inches of soil, but had no effect at the 3- to 6-inch depth.

The data in Table 4 show that no till tends to conserve soil organic matter and organic N near the soil surface, which agrees with results from a number of other studies (Blevins et al. 1983; Doran, 1980; Unger and McCalla, 1980). Haas et al. (1957) observed that loss of soil organic matter after 30 or more years of cultivation was generally greatest for those cropping or tillage practices that involved most soil disturbance (i.e., cultivated row crops and crop-fallow sequences). Such results, as well as the data in Table 4, suggest that cultivation is an oxidative process that hastens organic-matter decomposition. There are several reasons for this. First, the greater water content of the surface of no-tilled soils can enhance biomass production of both microorganisms and of crop plants. Second, crop residues for no-till are positioned above the soil surface where they are less subject to microbial decay; and third, the cooler soil temperatures of no till slow down oxidation rates.

In order to gain better insight into the effect of tillage practices on N cycling, ¹⁵N-depleted ammonium nitrate was applied at 40 LB N/acre to plots in wheat, and the fate of the tagged fertilizer was followed through the crop-fallow cycle to the second wheat crop. Periodically, soil and plant samples were collected and analyzed to follow movement of the N isotope through the various pools of the N cycle. The percent of the applied isotope found in several pools on various dates until harvest of the second crop is presented in Table 5.

The first sampling at tillering was about 3 weeks after fertilizer N was applied to the winter wheat. For the plow treatment, 48% of the isotope remained in the upper 4 inches as inorganic N compared to 36% for the no-till treatment. Generally, values for stubble mulch were between these extremes. Uptake by the wheat crop was 18 and 26% of the N applied for the plow and no-till treatments, respectively, with only traces found in either the visible or decaying (decomposed beyond point of recognition) crop residues. By harvest, 23 and 10% of the isotope remained in the upper 4 inches of soil as inorganic N for plow and no-till treatments, respectively, while 31 and 28% has been taken up by the whole plant (21 and 24%, respectively, in grain). By this time, up to 10% of the isotope had been immobilized in crop residues.

During the period of fallow, recovery of isotope as inorganic N in the upper 4 inches of plowed soil was near 0, compared to 6 or 7% for no-tilled soil. Also, only 1 to 2% of the isotope was in the crop residues for plowed fallow, compared to 9 to 12% for no tilled. During the second year of cropping, about 5 and 8% of the isotope applied was taken up by the second crop growing on plowed fallow and no-tilled fallow, respectively. Two and 3%, respectively, were removed in the grain of the second crop. Isotope in crop residues at harvest of the second crop was 2 and 6% of that applied for plowed and no-tilled fallow treatments, respectively. Thus, after harvest of the second crop, isotope in wheat straw, crop residues, and as soil inorganic N combined amounted to only 5% of that applied to plowed soil, compared to 13% for no tilled. Because most of the N in these forms would be in the readily labile or the relatively labile pools (Fig. 2), the amount of isotope potentially available to future crops from these sources would be greater for no tilled than for plowed soil. Also, no till usually had greater microbial biomass (Table 2), a readily labile source of N, which would add to N availability in no-tilled soil. Nitrogen in these pools may, therefore, account for the increase in potentially mineralizable N for no till compared to plowed soil (Table 4)

The growing plant integrates all the changes in the soil environment resulting from tillage or other management practices. Tillage-related changes can affect soil water and temperature regimes, alter microbiological activity and nutrient transformations, and directly influence plant root growth and activity. All of the above factors contribute in a major way to the growth and grain production of the crop.

Improved conservation of soil water resulting from reduced tillage has been documented in numerous publications (Unger and McCalla, 1980). Soil water data collected in this experiment support this conclusion in that no-tillage and reduced tillage usually increased soil water storage, compared to plowing. Typical data are shown in Table 6, and additional data for earlier years of this experiment are given by Fenster and Peterson (1979). Data in Table 6 shows that no-tilled soils typically contained at least 2 inches more soil water in the spring than did plowed soil. This difference generally persisted until the crop approached maturity (June 30). Thus, during the critical periods of tillering (May) and grain fill (June), wheat on no-tilled fallow usually had more soil water available than wheat on plowed fallow. However, lower soil temperatures for no-tilled soil could slow rate of crop growth and production.

Average yields for fertilized wheat were not affected by tillage (Table 7). Without N fertilization, yields for plow were unchanged, but yield for no till was reduced by 10% compared to fertilized no-till. Most of this reduction occurred on the set of plots harvested in 1980 and 1982, and may have been caused by inadequate control of downy brome (Bromus tectorum L.) on no-tilled plots. No weed-control problems were encountered on the other set of plots (harvested 1979 and 1981), which produced wheat yields slightly greater on no-tilled than on plowed fallow.

These results suggest that, if weed control is adequate, grain yields from no-tilled were at least equal to those on plowed fallow, and that an N response is more likely to occur on no-tilled fallow. It appears, therefore, that the extra water being conserved under no till is not effectively being translated into increased grain yield. Reduced soil temperatures may be one reason. Also, limited availability of N may be another cause. One might expect less available N in no-till because of the greater quantity of N immobilized as soil organic N and because the cooler soil temperature could resist mineralization rate. The fact that the percentage of the N taken up by wheat derived from fertilizer N was greater for wheat from no-tilled than that from plowed fallow (Table 5) suggests that the inorganic soil N pool in no-tilled soil contained less inorganic N mineralized from indigenous sources. Wheat grain harvested from fertilized no-tilled fallow had a lower protein content than that from plowed fallow, again suggesting less availability of N in no-tilled soil (Table 8).

Root growth was also greater for wheat grown on no-tilled than on plowed fallow (Table 9, from Wilhelm et al. 1982). This difference persisted at all sampling dates from early spring until anthesis in early June, even though root density increased 5- to 10-fold during this period. At all sampling dates, over 60% of the wheat roots were located in the upper foot of soil. Tillage treatments did not greatly affect rooting depth. Also, N fertilization had no effect on root density.

Data presented in this paper show how management decisions (especially tillage practices) alter soil environment and ultimately affect crop growth. These data from a tillage experiment on winter wheat at Sidney, Nebraska show how choice of tillage affect several soil physical properties that influence water, aeration, and temperature regimes. The effects of changes in these physical factors upon the number and activity of soil microorganisms have been documented, and the effects of microbial activity an N pool size and N fluxes have been presented. The net effect of the collective action of all these factors on wheat growth and production were then presented. Thus, data have been presented to show how the Agricultural Ecosystem model outlined in Fig.1 operated in this experiment.

From this model and the data presented, one can conclude that no-tilled, compared to plowed fallow, creates a wetter, cooler, less oxidative environment. This environment, along with physical protection of residues left on the soil surface, enhanced mobilization of N while slowing mineralization rates, and thereby increased the size of many of the soil organic N pools but often reduced inorganic soil N concentrations. Thus, wheat produced on no-tilled fallow generally had less water stress but greater N stress than wheat on plowed fallow. Although not studied in this experiment, such results suggest that placement of fertilizer N below the zone of organic-matter (and microbial biomass) enrichment in no-tilled soils may help reduce the immobilization of N and alleviate the greater N stress of no-tilled wheat.

Soil water and aeration relationships may best be expressed by using the concept of water-filled pore space (Table 1). Linn and Doran (1984) have shown that about 60% water-filled pore space is near optimum for aerobic activity. At lower values, water availability restricts biological activity, while at greater values, oxygen diffusion rate is limiting. Temperature and soluble C, the other environmental parameters important in controlling biological activity in soils, can be directly measured.

While results of this experiment help explain reasons for the variable plant growth response to tillage systems, one must also remember that factors other than soil water, aeration, temperature, and substrate availability can have a major impact on tillage effects on wheat production. Differential damage from diseases and insects has not been addressed, although we know that, under some conditions, these losses can be very significant. Also, mention was made of problems related to inadequate weed control, which can mask the effects of tillage on the soil environment. The model used in this paper, however, does appear to have utility in providing a framework in which changes in soil properties resulting from tillage can be interpreted in terms of potential plant growth and production.

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† From field experiments in Illinois, Kentucky, Minnesota and Nebraska.

.

† Poor control of downy brome.

A graft indicating the influence of tillage on the soil microenvironment as related to microbial activities, nitrogen transformations and plant growth as well as a graft of the nitrogen cycle in the soil-plant ecosystem (from Campbell et al., 1976) was included in the original proceedings.