Desert leaf litter decay: Coupling of microbial respiration, water‐soluble fractions and photodegradation

Abstract

The mechanisms of plant litter decay in drylands are poorly understood, limiting the accuracy of nutrient-cycling models for these systems. We monitored the decay of 12 leaf litter types on the soil surface of the Sonoran Desert for 34 months and assessed what traits predicted mass loss and how exposure to different wavebands of sunlight influenced mass loss. Mass loss varied considerably among litter types, ranging from 42%-96% after 34 months in full sunlight. Traditional indices of litter quality (e.g., initial C:N or lignin:N ratios) failed to predict differences in mass loss among litter types. The strongest predictor of mass loss was the microbial respiration rate of initial litter, which explained 45%-54% of the variation in loss among litter types. Microbial respiration rates were not correlated with traditional indices of litter quality, but were positively correlated with the water-soluble fraction in litter and concentrations of dissolved organic C in this fraction. Traditional indices of litter quality failed to predict decay likely because they did a poor job of predicting microbial degradability of litter, not because microbial degradation was a minor driver of decay. In all radiation-exposure treatments, water-soluble fractions and respiration rates increased through decay and were several times higher after 34 months than initially. Hence, labile pools and microbial degradability of litter increased through decay in contrast to traditional views that labile pools decline and constrain microbes. Litter exposed to UV or UV through blue radiation wavelengths, lost on average 1.3 times or 1.5 times more mass, respectively, than litter not exposed to these wavebands. The magnitude of this photodegradation was greater in litter types that had higher initial concentrations of hemicellulose and cellulose per unit surface area. Litter exposed to full sun had higher water-soluble fractions and usually had higher respiration rates, illustrating that sunlight accelerated microbial degradation by increasing labile pools. The processes driving litter decay appeared to differ appreciably from mesic systems and involved strong couplings between abiotic and biotic drivers.

Thermal abiotic emission of CO2 and CH4 from leaf litter and its significance in a photodegradation assessment

“…Indirect evidence corroborating our idea that thermal emission did not contribute to the greater C loss of litter in full sun comes from the contrasting temporal patterns of thermal emission vs. photodegradation: The contribution of thermal emission declined appreciably as litter aged, whereas the magnitude of photodegradation increased over this field experiment (Day et al. ), as well as a past experiment at the same site (Day et al. ).…”

“…Our findings strongly suggest that thermal emission did not confound our field photodegradation assessment or prior conclusion that photodegradation was a significant driver of litter mass or C loss (Day et al. ). Not only did thermal emission represent a very modest pathway for C loss from litter, but cumulative emission was actually greater from litter in the No UV/blue than the full sun treatment over the experiment.…”

“…Some studies have documented elevated temperatures under transparent filters, particularly during summer periods (Austin and Vivanco , Day et al. , ), although many have reported no significant temperature differences, which we suspect stems in some cases from small replication size. In any case, the magnitude of these elevated temperatures would depend on the specific filters used (e.g., which wavebands and how much solar radiation was filtered) and their design (e.g., height and shape of filters and their influence on air mixing).…”

“…In a recent study, we identified what litter traits predicted the litter mass loss of 12 leaf litter types and assessed how exposure to solar radiation influenced mass loss over 34 months in the Sonoran Desert (Day et al. ). In brief, we found that litter microbial respiration rates were the strongest predictor of litter mass loss.…”

“…For example, differences in decay rates among litter types are usually well predicted by indices of litter decomposability by microbes (which we hereafter refer to as litter quality), such as C:N or lignin:N ratios (Meentemeyer 1978, Melillo et al 1982, Cornwell et al 2008. A notable exception is drylands, where these indices fail to predict differences in decay among litter types (Schaefer et al 1985, Cepeda-Pizarro and Whitford 1990, Vanderbilt et al 2008, Day et al 2018, Liu et al 2018. In many terrestrial ecosystems, decay rates are well predicted by models that incorporate these litterquality indices together with climatic indices that estimate microbial activity.…”