Burrowing detritivores regulate nutrient cycling in a desert ecosystem

Abstract

Nutrient cycling in most terrestrial ecosystems is controlled by moisture-dependent decomposer activity. In arid ecosystems, plant litter cycling exceeds rates predicted based on precipitation amounts, suggesting that additional factors are involved. Attempts to reveal these factors have focused on abiotic degradation, soil–litter mixing and alternative moisture sources. Our aim was to explore an additional hypothesis that macro-detritivores control litter cycling in deserts. We quantified the role different organisms play in clearing plant detritus from the desert surface, using litter baskets with different mesh sizes that allow selective entry of micro-, meso- or macrofauna. We also measured soil nutrient concentrations in increasing distances from the burrows of a highly abundant macro-detritivore, the desert isopod Hemilepistus reaumuri. Macro-detritivores controlled the clearing of plant litter in our field site. The highest rates of litter removal were measured during the hot and dry summer when isopod activity peaks and microbial activity is minimal. We also found substantial enrichment of inorganic nitrogen and phosphorous near isopod burrows. We conclude that burrowing macro-detritivores are important regulators of litter cycling in this arid ecosystem, providing a plausible general mechanism that explains the unexpectedly high rates of plant litter cycling in deserts.

Arthropods as the Engine of Nutrient Cycling in Arid Ecosystems

“…For example, Sagi et al (2019) found a 1.5-fold increase in ammonium, a twofold increase in nitrate, and a 1.3-fold increase in phosphate concentrations near the vertical burrow of desert isopods (Hemilepistus reaumuri) compared with 20 cm away from the burrow. Notably, nutrient enrichment was not limited to the burrow walls but extended up to a distance of about 10 cm from the vertical burrows [25]. In another example, Nutting et al (1987) measured an 11-fold increase in ammonium and a twofold increase in phosphate in soil brought to the surface by Sonoran desert termites compared to adjacent surface soil [49].…”

“…In the Chihuahuan desert, termites are responsible for 50% of the leaf litter removal [11,22,23], and in the hot and dry Baza basin detritivores increased plant litter mass loss by a factor of 1.23 [24]. In the Negev desert macro-detritivores accounted for 89% of the litter removal, and the highest removal rate was observed in the dry and hot summer when the contribution of microorganisms and mesofauna was negligible [25].…”

“…In many deserts, animals cannot construct stable burrows in the loose organic-rich soil beneath shrub canopies [67,68]. Thus, many macro-arthropods transport nutrients from the macrophytic patches to their burrows, creating a nutrient-rich patch around the burrow's entrance [25,49,69]. Consequently, macro-arthropod activity may create a more complex spatial mosaic of nutrient distribution [70,71], or blur the distinct differences in nutrient concentration between patch types.…”

“…For instance, desert isopods evacuate soil, fecal pellets, and uneaten detritus from their burrow and mound them around the entrance. These loose mounds are much richer in nutrients, with a 1.5-fold increase in ammonium and a 9.8-fold increase in nitrate compared with adjacent soil crust [25]. In the Chihuahuan desert, the large differences in soil nitrogen between shrub patches and interspaces were canceled by termite activity [72].…”

Detritivore conversion of litter into faeces accelerates organic matter turnover

“…soil animals that feed on decomposing litter, assimilate a part of it and typically return the largest part to the soil as faeces ('litter conversion into detritivore faeces' hereafter) 6 . Studies from temperate 7 , Mediterranean 8 , tropical 9 and arid 10 ecosystems reported that large portions of annual litterfall are consumed by detritivores and converted into faeces. This conversion entails physical changes with a reduction of initially intact litter to minute particles that constitute the faeces (referred to as comminution or fragmentation in the literature 2 ) and chemical changes with a partial digestion of the litter passing through detritivore guts 6 .…”