Ecological Consequences of Mesquite Fixation of Nitrogen

While the importance of mesquite-fixed nitrogen has not been demonstrated in South Texas, there is no reason to doubt its importance. With that assumption, it is possible to construct a scenario which is consistent with a large number of the observations by ranchers, range scientists, and ecologists in the Rio Grande Plain.

First, it is easy to understand why grass production is elevated with mesquite removal, especially near centers of clumps (Jacoby et al., 1982). The nitrogen contained in the root-mycorrhizal association beneath the shrub clump and in the litter beneath is suddenly released to grasses and other plants, with grasses already growing beneath responding most readily (Jacoby et al., 1982). (Crystal City spinach farmers early in this century got their best production from freshly cleared mesquite patches [W. Averhoff, personal communication]). Medium term (8 year) experiments on range grasses interplanted with herbaceous legumes such as Medicago and Astragalus (McGinnies and Townsend, 1983) indicate increased forage production and protein content over grasses grown alone. Contrary to expectation, these workers found no increase in soil nitrogen in the legume treatment area. Unfortunately in this study, only the top 15cm of soil was analyzed, yet all grasses and legumes involved are rooted at depths well below 15cm. Indeed in this case, the release of legume-fixed nitrogen as roots decay should be expected between 5 to 20ft deep, since both legumes and grasses (Agropyron species) involved, are sending roots to that zone (Weaver and Clements, 1938). Likewise, we should anticipate similar patterns of deep lateral exchange in the mesquite system based on the Sonoran Desert evidence summarized above.

Second, the "invasion" by mesquite of overgrazed pasture or heavily farmed land, is a consequence of mesquite's ability to fix nitrogen. In semi-arid regions, nitrogen is lost by ammonia volatilization or denitrification directly to the atmosphere in relative amounts estimated to rival (in terms of percent of fixed nitrogen in the system) nitrate losses by leaching in more mesic environments (West and Skujins, 1978). The denitrification process is enhanced by exposure of soil to the high temperatures of direct sun, by release of mineralized nitrogen and phosphorous from closed root-mycorrhizal systems, and by increased organic carbon sources for denitrifying bacteria, all consequences of harsh methods of shrub control. Later, after a period of overgrazing, the soil of the range becomes further depleted with respect to nitrogen, and nitrogen-fixing plants such as mesquite enjoy a competitive advantage over non-fixing plants such as grasses, and return denser than ever. This initiates secondary succession toward a less diverse and denser "climax" than was originally present. Mesquite colonizes land as "unproductive as an old stove lid," it doesn't create it!

Short term experiments on mesquite seedlings (Ueckert et al., 1979) do suggest that competition with herbaceous vegetation (including grasses) can substantially decrease seedling establishment. Experiments in true prairie ecosystems have examined the role of nitrogen and moisture on the competition between nitrogen-fixing legumes and grasses (Dodd and Lauenroth, 1979; Lauenroth and Dodd, 1979). Plots were enriched with nitrogen, water, water plus nitrogen, or not treated (control) and biomass production followed for six years. Not surprisingly, grasses dominate the standing crop in all plots, but legumes increase by a factor of 30 to 40 times in "water only" treatments because of their competitive ability under low nitrogen levels.

Similar studies are required for the Rio Grande Plain. However, to be maximally useful, such experiments should also include

1. manipulation of the Rhizobium-mesquite interaction (nodulated vs. unnodulated treatments)
2. grazing and or browsing exclusion treatments
3. burning
4. other nutrients such as phosphorus

Each of these factors can and does vary in the natural system along with water and nitrogen availability. It will be a complex and labor-intensive process to tease apart the role of these six obvious factors across a representative range of soil types. However, if such results were available, the methods of manipulating shrub density employed by ranchers might evolve in totally unexpected directions and become more compatible with retention of natural diversity in the region. Nitrogen, added as nitrate during wet periods might actually retard the establishment of mesquite in these nitrogen-poor soils by inhibiting nodulation (Gibson, 1976) of mesquite seedlings, while favoring the growth of grasses and other herbaceous competitors.

Ironically, ranchers in South Texas who root-plow mesquite on an 8-15 year cycle are, under this view of the system, simply using mesquite as a long-lived cover crop, much as Acacia arabica was rotated with grain sorghum in the Sahel of Africa (8 year cycle) by primitive farmers (see Kassas, 1970). Does it make a difference then, whether we recognize mesquite and other nitrogen fixing woody plants as beneficial cover crops or view them as noxious weeds? The answer is that is makes a vast difference to the kind of science we will conduct on this system; the beneficial hypothesis will lead us to explore the details of the nitrogen cycle and the biology of organisms involved in its operation. Many more natural species in the system may prove to be important to its sustained productivity. We may find that low overhead, high profit-margin ranching is not only compatible with conservation of natural diversity in the region, but also dependent upon it for long term stability. Studies such as those in the Sahel of Africa (Breman and de Wit, 1983) are needed in the Tamaulipan Province.