Amy T. Austin

Water pulses and biogeochemical cycles in arid and semiarid ecosystems

The episodic nature of water availability in arid and semiarid ecosystems has significant consequences on belowground carbon and nutrient cycling. Pulsed water events directly control belowground processes through soil wet-dry cycles. Rapid soil microbial response to incident moisture availability often results in almost instantaneous C and N mineralization, followed by shifts in C/N of microbially available substrate, and an offset in the balance between nutrient immobilization and mineralization. Nitrogen inputs from biological soil crusts are also highly sensitive to pulsed rain events, and nitrogen losses, particularly gaseous losses due to denitrification and nitrate leaching, are tightly linked to pulses of water availability. The magnitude of the effect of water pulses on carbon and nutrient pools, however, depends on the distribution of resource availability and soil organisms, both of which are strongly affected by the spatial and temporal heterogeneity of vegetation cover, topographic position and soil texture. The ‘inverse texture hypothesis’ for net primary production in water-limited ecosystems suggests that coarse-textured soils have higher NPP than fine-textured soils in very arid zones due to reduced evaporative losses, while NPP is greater in fine-textured soils in higher rainfall ecosystems due to increased water-holding capacity. With respect to belowground processes, fine-textured soils tend to have higher water-holding capacity and labile C and N pools than coarse-textured soils, and often show a much greater flush of N mineralization. The result of the interaction of texture and pulsed rainfall events suggests a corollary hypothesis for nutrient turnover in arid and semiarid ecosystems with a linear increase of N mineralization in coarse-textured soils, but a saturating response for fine-textured soils due to the importance of soil C and N pools. Seasonal distribution of water pulses can lead to the accumulation of mineral N in the dry season, decoupling resource supply and microbial and plant demand, and resulting in increased losses via other pathways and reduction in overall soil nutrient pools. The asynchrony of resource availability, particularly nitrogen versus water due to pulsed water events, may be central to understanding the consequences for ecosystem nutrient retention and long-term effects on carbon and nutrient pools. Finally, global change effects due to changes in the nature and size of pulsed water events and increased asynchrony of water availability and growing season will likely have impacts on biogeochemical cycling in water-limited ecosystems.

Plant litter decomposition in a semi-arid ecosystem controlled by photodegradation

The carbon balance in terrestrial ecosystems is determined by the difference between inputs from primary production and the return of carbon to the atmosphere through decomposition of organic matter. Our understanding of the factors that control carbon turnover in water-limited ecosystems is limited, however, as studies of litter decomposition have shown contradictory results and only a modest correlation with precipitation. Here we evaluate the influence of solar radiation, soil biotic activity and soil resource availability on litter decomposition in the semi-arid Patagonian steppe using the results of manipulative experiments carried out under ambient conditions of rainfall and temperature. We show that intercepted solar radiation was the only factor that had a significant effect on the decomposition of organic matter, with attenuation of ultraviolet-B and total radiation causing a 33 and 60 per cent reduction in decomposition, respectively. We conclude that photodegradation is a dominant control on above-ground litter decomposition in this semi-arid ecosystem. Losses through photochemical mineralization may represent a short-circuit in the carbon cycle, with a substantial fraction of carbon fixed in plant biomass being lost directly to the atmosphere without cycling through soil organic matter pools. Furthermore, future changes in radiation interception due to decreased cloudiness, increased stratospheric ozone depletion, or reduced vegetative cover may have a more significant effect on the carbon balance in these water-limited ecosystems than changes in temperature or precipitation.

The N-15 natural abundance (delta N-15) of ecosystem samples reflects measures of water availability

We assembled a globally-derived data set for site-averaged foliar delta(15)N, the delta(15)N of whole surface mineral soil and corresponding site factors (mean annual rainfall and temperature, latitude, altitude and soil pH). The delta(15)N of whole soil was related to all of the site variables (including foliar delta(15)N) except altitude and, when regressed on latitude and rainfall, provided the best model of these data, accounting for 49% of the variation in whole soil delta(15)N. As single linear regressions, site-averaged foliar delta(15)N was more strongly related to rainfall than was whole soil delta(15)N. A smaller data set showed similar, negative correlations between whole soil delta(15)N, site-averaged foliar delta(15)N and soil moisture variations during a single growing season. The negative correlation between water availability (measured here by rainfall and temperature) and soil or plant delta(15)N fails at the landscape scale, where wet spots are delta(15)N-enriched relative to their drier surroundings. Here we present global and seasonal data, postulate a proximate mechanism for the overall relationship between water availability and ecosystem delta(15)N and, newly, a mechanism accounting for the highly delta(15)N-depleted values found in the foliage and soils of many wet/cold ecosystems. These hypotheses are complemented by documentation of the present gaps in knowledge, suggesting lines of research which will provide new insights into terrestrial N-cycling. Our conclusions are consistent with those of Austin and Vitousek (1998) that foliar (and soil) delta(15)N appear to be related to the residence time of whole ecosystem N.

Dual role of lignin in plant litter decomposition in terrestrial ecosystems

Plant litter decomposition is a critical step in the formation of soil organic matter, the mineralization of organic nutrients, and the carbon balance in terrestrial ecosystems. Biotic decomposition in mesic ecosystems is generally negatively correlated with the concentration of lignin, a group of complex aromatic polymers present in plant cell walls that is recalcitrant to enzymatic degradation and serves as a structural barrier impeding microbial access to labile carbon compounds. Although photochemical mineralization of carbon has recently been shown to be important in semiarid ecosystems, litter chemistry controls on photodegradative losses are not understood. We evaluated the importance of litter chemistry on photodegradation of grass litter and cellulose substrates with varying levels of lignin [cellulose-lignin (CL) substrates] under field conditions. Using wavelength-specific light attenuation filters, we found that light-driven mass loss was promoted by both UV and visible radiation. The spectral dependence of photodegradation correlated with the absorption spectrum of lignin but not of cellulose. Field incubations demonstrated that increasing lignin concentration reduced biotic decomposition, as expected, but linearly increased photodegradation. In addition, lignin content in CL substrates consistently decreased in photodegradative incubations. We conclude that lignin has a dual role affecting litter decomposition, depending on the dominant driver (biotic or abiotic) controlling carbon turnover. Under photodegradative conditions, lignin is preferentially degraded because it acts as an effective light-absorbing compound over a wide range of wavelengths. This mechanistic understanding of the role of lignin in plant litter decomposition will allow for more accurate predictions of carbon dynamics in terrestrial ecosystems.

Methods of Estimating Aboveground Net Primary Productivity

Estimating net primary productivity (NPP) has been a central goal of basic and applied ecologists. Very important questions rely on good estimates of NPP: the global carbon balance, the location of the missing carbon sink, and predictions of global climate change (see Chapter 3). Primary productivity represents the major input of carbon and energy into ecosystems and McNaughton (1989) proposed NPP as an integrative variable of the functioning of the whole ecosystem because of its relationships with animal biomass, secondary productivity, and nutrient cycling. From an applied perspective, primary production of grasslands determines forage availability and constrains animal carrying capacity whereas primary production of forests is directly related to wood yield.

Carbon and nitrogen dynamics across a natural precipitation gradient in Patagonia, Argentina

Abstract Both ecosystem carbon gain and nutrient availability are largely constrained by the magnitude and seasonality of precipitation in arid and semi-arid ecosystems. We investigated the role of precipitation on ecosystem processes along an International Geosphere Biosphere Programme (IGBP) transect in temperate South America. The transect consists of a contiguous precipitation gradient in the southern region of Argentinean Patagonia (44–45° S), from 100 mm to 800 mm mean annual precipitation (MAP) and vegetation ranging from desert scrub to closed canopy forest. Gravimetric soil water content tracked changes in seasonal and annual precipitation, with a linear increase in soil water content with increasing MAP. Above-ground net primary production (ANPP) increased linearly along the gradient of precipitation (ANPP = − 31.2 + 0.52 MAP, r2 = 0.84, p = 0.028), supporting the relationship that carbon assimilation is largely controlled by available water in these sites, and was in general agreement with regional models of ANPP and rainfall. However, inorganic soil nitrogen was also highly linearly correlated with both MAP ([N] = 0.19 MAP − 32, r2 = 0.96, p = 0.003) and ANPP (ANPP = 2.6 [Ninorganic]+59.4, r2 = 0.79, p = 0.042), suggesting a direct control of precipitation on nitrogen turnover and an interaction with nitrogen availability in controlling carbon gain. The asynchrony of precipitation and changes in dominant vegetation may play important roles in determining the carbon-nitrogen interactions along this rainfall gradient. Nomenclature: Correa (1971); Dimitri (1972). Abbreviations: ANPP = Above-ground net primary production, IGBP = International Geosphere Biosphere Programme; MAP = Mean annual precipitation.

Responses and feedbacks of coupled biogeochemical cycles to climate change: examples from terrestrial ecosystems

The biogeochemical cycles of carbon (C), nitrogen (N), and phosphorus (P) are fundamental to life on Earth. Because organisms require these elements in strict proportions, the cycles of C, N, and P are coupled at molecular to global scales through their effects on the biochemical reactions controlling primary production, respiration, and decomposition. The coupling of the C, N, and P cycles constrains organismal responses to climatic and atmospheric change, suggesting that present-day estimates of climate warming through the year 2100 are conservative. N and P supplies constrain C uptake in the terrestrial biosphere, yet these constraints are often not incorporated into global-scale analyses of Earths climate. The inclusion of coupled biogeochemical cycles is critical to the development of next-generation, global-scale climate models.

Differential Controls of Water Input on Litter Decomposition and Nitrogen Dynamics in the Patagonian Steppe

Studies of the effects of precipitation on litter decomposition and nitrogen mineralization in arid and semiarid environments have demonstrated contradictory results. We conducted a manipulative experiment with rainout shelters in the semiarid Patagonian steppe, aimed at assessing the direct effects of water availability on litter decomposition and net nitrogen mineralization while isolating the indirect effects. We created four levels of precipitation input: control and three levels (30, 55 and 80%) of precipitation interception and we examined litter decomposition and nutrient release of a dominant grass species, Stipa speciosa, inorganic soil nitrogen, and in situ net nitrogen mineralization over two consecutive years. Litter decomposition rates (k, year−1) varied significantly (P < 0.001) among precipitation interception treatments and were positively correlated with incoming annual precipitation (APPT, mm/year) (k = 0.0007 × APPT + 0.137). In contrast, net N mineralization was not correlated with incoming precipitation. Soil NO3− significantly decreased with increasing precipitation input, whereas soil NH4+ concentration did not differ among precipitation interception treatments. Controls of water input on litter decomposition appear to be different from controls on N mineralization in the semiarid Patagonian steppe. We suggest that although water availability affects both the mineralization of C and N, it differentially affects the movement and fate of the inorganic products. A consequence of the accumulation of inorganic N during dry episodes is that periods of maximum water and soil nutrient availability may occur at different times. This asynchrony in the availability of N and water in the soil may explain the observed lags in the response of primary production to increases in water availability.

Has water limited our imagination for aridland biogeochemistry

The classic ecological paradigm for deserts, that all processes are controlled by water availability, has limited our imagination for exploring other controls on the cycling of carbon and nutrients in aridland ecosystems. This review of recent studies identifies alternative mechanisms that challenge the idea that all soil processes in aridlands are proximately water-limited, and highlights the significance of photodegradation of aboveground litter and the overriding importance of spatial heterogeneity as a modulator of biotic responses to water availability. Aridlands currently occupy >30% of the terrestrial land surface and are expanding. It is therefore critical to incorporate these previously unappreciated mechanisms in our understanding of aridland biogeochemistry to mitigate the effects of desertification and global change.

Canopy Light and Plant Health

During the course of this century, increasing human population and economic development will continue to put pressure on agricultural systems for increased crop yields ([Rosegrant and Cline, 2003][1]; [Foley et al., 2005][2]). Grains produced for livestock feed and biofuels are now competing for