Leaf litter identity alters the timing of lotic nutrient dynamics

Abstract

The effects of resource quality on ecosystems can shift through time based on preferential use and elemental needs of biotic consumers. For example, leaf litter decomposition rates are strongly controlled by initial litter quality, where labile litter is processed and depleted more quickly than recalcitrant litters. We examined the effect of this processing continuum on stream nutrient dynamics. We added one of four different litter compositions differing in litter quality (cottonwood [Populus deltoides], labile; sycamore [Platanus occidentalis], recalcitrant; bur oak [Quercus macrocarpa], recalcitrant; and mixed [equivalent mixture of previous three species]) to 12 large (c. 20 m long, with riffle, glide and pool sections) outdoor stream mesocosms to assess the effect of litter species composition on whole‐stream nutrient uptake. Nutrients were dosed once weekly for 8 weeks to measure uptake of NH4–N, NO3–N, and PO4–P. We also measured changes in litter C, N, and P content on days 28 and 56 of the study. Nutrient uptake rates were highly variable, but occasionally very different among litter treatments (c. 5× between highest and lowest uptake rates by species). Uptake rates were generally greatest in cottonwood (labile) streams early in the study. However, during the last 4 weeks of the study, bur oak streams (recalcitrant) took up more nutrients than cottonwood streams, resulting in more cumulative NO3–N uptake in bur oak than in cottonwood streams. Cumulative NO3–N uptake was greater in mixed streams than expected (non‐additive) on two dates of measurement, but was generally additive. Changes in litter nutrient content largely corroborated nutrient uptake patterns, suggesting strong N immobilisation early in the study and some N mineralisation later in the study. P was strongly retained by most litters, but especially bur oak. Nutrient content of litter also largely changed additively, suggesting minimal evidence for non‐additive diversity effects on nutrient source/sink status. Our results demonstrate that litter species identity can have whole‐ecosystem effects on stream nutrient dynamics, with important implications for the form and fate of nutrients exported downstream. Further, diverse litter assemblages may serve as temporal stabilisers of ecosystem processes, such as nutrient sequestration, due to microbial nutrient requirements and differential decomposition rates, or the classic litter processing continuum.

Soil C, N, P and K stoichiometry affected by vegetation restoration patterns in the alpine region of the Loess Plateau, Northwest China

“…After the implementation of the Grain-for-Green project, the litter of perennial vegetation returned to the soil, thereby increasing soil nutrients [ 48 ]. The contents of C and N are higher in litter [ 49 ], P is not easily decomposed [ 50 ], and water-soluble potassium is easily adsorbed and fixed on soil clay particles [ 48 ]; thus, a large increase in the SOC, TN, AN and AK contents was observed in the soil. Among the different land use types, FL had the highest litter biomass; therefore, it returned the most nutrients to the soil.…”

Global Patterns and Controls of Nutrient Immobilization on Decomposing Cellulose in Riverine Ecosystems

“…In riparian zones, N IMM on cotton was similar to rates measured on six leaf litter species (20–33 μg N g −1 C d −1 ; Aber & Melillo, 1982) but lower than those for crop residues in agricultural soils (462 μg N g −1 C d −1 ; Recous et al., 1995). In rivers, N IMM on cotton was within the range of immobilization on natural litter measured either by N accumulation in residual litter (Robbins et al., 2019) or isotopic dilution in microbial pools (Cheever et al., 2013; Pastor et al., 2014). P immobilization on decomposing litter is rarely measured, but our rates of P IMM on cotton were similar to those recorded for three litter types in an artificial stream (0–12 μg P g −1 C d −1 ; Robbins et al., 2019).…”

“…However, standardized organic matter substrates are commonly used at small spatial scales along narrow nutrient gradients (e.g., Cheever et al, 2013;Pastor et al, 2014) and the only large-scale study using standardized substrates did not directly measure microbial uptake (Woodward et al, 2012). Few studies have quantified both N and P immobilization within the same decomposing substrate (but see Robbins et al, 2019) thus we have limited evidence about the stoichiometry of immobilization. Bacteria (inclusive of all decomposer prokaryotes) and fungi are the primary decomposers of plant litter, and both taxa can produce biomass with an N:P reflective of the nutrient supply (Danger & Chauvet, 2013;Godwin & Cotner, 2015).…”

Pathways, Mechanisms, and Consequences of Nutrient-Stimulated Plant Litter Decomposition in Streams

“…Managing nutrient pollution requires consideration of how leaf litter in streams contributes to ecosystem functions that benefit and support ecosystem health and human well-being (i.e., ecosystem services; Frainer et al [Chapter 21 in this volume], Richardson et al [Chapter 22 in this volume]); with the understanding that nutrient enrichment may modify the availability (timing, retention and export) of leaf litter resources that fuel stream food webs, as well as stream nutrient uptake rates and export (Newbold, Elwood, O'Neill et al, 1983;Robbins et al, 2019), and feedbacks to the global C cycle and climate change ). Models and observations that target site-to catchment-scale understanding of how nutrients speed the sequence of leaf litter depletion from annual peaks to annual minima could provide several useful benchmarks that link stream leaf litter to its important roles as a driver of other critical stream ecosystem functions.…”