Plant-Feeding Nematodes in Deep-Rooting Desert Ecosystems

In the Chihuahuan Desert of southern New Mexico, mesquite (Prosopis glandulosa) occurs in a variety of landscape positions where water may accumulate to varying depths. The structure of Chihuahuan Desert landscapes has changed dramatically in the past 100 yr with mesquite expanding from its original distribution (playa, arroyo) into grassland and dune ecosystems. Our objective was to examine spatial and seasonal distribution of plant—feeding nematodes and the potential importance of herbivory in four mesquite communities (playa, arroyo, dunes, grassland) hypothesized to differ in rooting depth and distribution. These sites were compared to a more shallow—rooted creosote bush (Larrea tridentata) community. Intact soil cores were recovered to depths of 13 m beneath the canopies of mesquite or creosote bush using a continuous sampling tube drilling system in the winter, spring, and fall on the Jornada LTER site, Las Cruces, New Mexico. Nematode density and root mass decreased with depth. Nematodes were found to the maximum depth of recovered roots only at the playa (11—12 m) and occurred as deep as 5—6 m at the arroyo, 2—3 m at the dunes, and 1—2 m at the grassland and creosote bush sites. At the playa, 75% of the nematodes and 90% of the roots were below 0.5 m, whereas, in the creosote bush system 79% of the roots and 99.7% of the nematodes were recovered in the upper 0.5 m of soil. Nematode diversity (seven genera) and endoparasite densities were greatest at the playa. Only ectoparasites (three genera) were found at the creosote bush system. Total nematodes across all sites were highly correlated with total soil N, NH4+, and PO4—3, but not NO3—. Nematodes were not positively correlated with soil moisture. Potential herbivory was best described by nematode numbers per gram root or an herbivory index (based on nematode density weighted by an impact factor for each genus), rather than the ratio of endoparasite:ectoparasite numbers. The herbivory index decreased in the order playa > creosote bush > dune > grassland > arroyo. The movement of mesquite into new habitats (grassland, dunes) is associated with shallow rooting, and relative to the playa, a greater allocation of roots to the upper 0.5 m soil layer, a decrease in nematode herbivory and diversity, and, a failure of the nematodes to be distributed throughout the entire rooting zone. The densities of ecto— and endoparasites varied with site, thus, plant—feeding nematodes should not be treated as a single guild since they have different effects on roots. Our study shows that in some ecosystems the majority of herbivory may occur at soil depths rarely studied. Although technically difficult to examine in ecosystems where deep—rooted plants occur, the potential importance of deep soil biota and their effects on plant growth and nutrient cycling should be considered.