Microsite location and litter quality influence shrub and grass litter decomposition in two drylands in the southwestern United States

Background/Questions/Methods

Encroachment of leguminous shrubs into areas historically dominated by grasses can alter carbon (C) storage by influencing litter decomposition.  Shrub canopies may alter the understory microclimate relative to canopy interspace areas.  In addition, nitrogen (N)-rich shrub leaf tissue may enhance decomposition of grass litter by provide a source of available N to microbes. The objectives of our study were to quantify whether 1) shrub microsite influences litter decomposition and 2) N-rich shrub litter enhances decomposition of native grass litter.

We conducted a field litter decomposition experiment in two dryland systems where shrub encroachment has occurred over the past century: Santa Rita Experimental Range (SRER) in Arizona and Jornada Basin Long-Term Ecological Research Site (JRN) in New Mexico.  Senesced leaf tissue was collected at each site from one shrub and two grass species and incubated in litter bags either as single or mixed species (shrub + grass) treatments.  At each site, litter bags were placed in two microenvironments (under shrub canopies and in interspace areas).  Bags were retrieved at 6 months and litter was analyzed for percent remaining of initial mass, C, and N (IMR, ICR, INR, respectively). 

Results/Conclusions

After 6 months of incubation, litter lost 20-60% and 30-70% of its initial mass at the JRN and SRER, respectively.  Microenvironment and species type influenced decomposition at both sites.  At JRN, IMR and INR were higher under canopies than interspaces (IMR: F_1,30=4.2, p=0.05; INR: F_1,30=28.9, p<0.001).  At the SRER, only INR showed differences with microsite (F_1,28=16.4, p<0.001) and was higher under canopies than interspaces.  IMR, ICR, and INR were lower for shrub than grass litter regardless of microsite (JRN-IMR: F_2,30=291.5, p<0.0001; ICR: F_2,20=148.9, p<0.0001; INR: F_2,30=54.1, p<0.0001; SRER-IMR: F_2,28=8.2, p<0.01; ICR: F_2,28=6.6, p<0.01; INR: F_2,28=7.0, p<0.01) and followed expected patterns given initial litter quality.  Shrub litter had no direct effect on grass decomposition when decomposed together.  However, preliminary results suggest that shrub canopies indirectly affect litter decomposition by altering microenvironment.  Mass and C loss from decomposing litter was reduced under shrub canopies.  At both sites, grass litter immobilized N when incubated under shrub canopies and mineralized N when incubated in interspaces.  In conclusion, shrub canopies slowed down decomposition of shrub and grass litter.