Mesquite and the Nitrogen Cycle

There are other reasons besides the lack of long term studies to suggest the "beneficial" hypothesis. The most important consideration is that of nitrogen, an abundant element in the atmosphere, but unavailable to most organisms unless first converted to nitrate (NO3) or ammonia (NH4). These compounds are both rare in soils of semi-arid regions. Nitrogen is both a major limiting factor to plant productivity, and a critical building block of beef protein. Thanks to studies initiated by IBP (International Biological Program) in arid regions, quantitative data are now available on the role of mesquite in the nitrogen cycle of semi-arid and arid regions of the southwestern United States. Initial studies of nutrient distribution have revealed significant accumulation of nitrogen and carbon under mesquite (Barth and Klemmedson (1978;1982)[Figure 1].

Those of us who have long suspected a significant nitrogen fixing ability of mesquite based on the lushness and dark green color of grasses and other plants growing in mesquite clumps (Plate 3), have had suspicions confirmed. For example, the studies of Jarrell and colleagues (Rundel et al., 1982; Virginia et al., 1982; Virginia and Jarrell, 1983; Jarrell et al., 1983 in press); Nilsen et al., 1983 in press, and personal communication), in the Sonoran Desert have demonstrated that mesquite is nodulated (but at depth), that it does fix nitrogen, that nitrogen accumulates in the soil of the mesquite clump, especially as nitrate and at concentrations approaching well fertilized cropland (Table 1 below), and that the pool of soil nitrogen in the system (500 years of succession on a dry lake bed) can be accounted for as an accumulation of yearly increments due to mesquite fixation alone.

Table 1. Total nitrogen, organic nitrogen, and nitrate nitrogen (NO3-N) in soil of a mesquite stand: Sonoran Desert of California vs. other systems. (from Nilsen et al., 1983, Table 4)
CommunityNO3-N
(g/m2)		Total N
(g/m2)		%NO3-N
Mesquite
under253102025
between5316034
Conifer forest0.24500.04
Grassland0.013520.01
Mojave Desert3993

Unfortunately we are almost totally ignorant of the quantitative or even qualitative details of the nitrogen cycle in the Tamaulipan biota of South Texas. However, we must consider woody legumes like mesquite as major contributors to the nitrogen pool upon which grass and ranching depends. Such a conclusion would certainly not be forthcoming from a cursory examination of freshly root-plowed mesquite. Typically, no nodules are to be seen. But on reflection that should be expected. Nodulation is inhibited by the high nitrate levels around older trees (is this one reason seedling mesquites are rare near parent trees?) and probably by the dry surface conditions. Indeed, in the Sonoran Desert, Jarrell's group found few or no nodules until the water table was reached. Mesquite is deeply rooted (up to 53m or 172ft[!]; Phillips, 1963) and is therefore green and productive of new nitrogen even during drought (Nilsen et al., 1983). Mesquite nitrogen, also fixed when grasses are abundant, becomes available later as mesquite litter is decomposed. I should also emphasize the role of such deeply rooted plants in bringing other nutrients to the surface.