Abstract
Anthropogenic nitrogen (N) deposition and resulting differences in ecosystem N and phosphorus (P) ratios are expected to impact photosynthetic capacity, that is, maximum gross primary productivity (GPPmax ). However, the interplay between N and P availability with other critical resources on seasonal dynamics of ecosystem productivity remains largely unknown. In a Mediterranean tree-grass ecosystem, we established three landscape-level (24ha) nutrient addition treatments: N addition (NT), N and P addition (NPT), and a control site (CT). We analyzed the response of ecosystem to altered nutrient stoichiometry using eddy covariance fluxes measurements, satellite observations, and digital repeat photography. A set of metrics, including phenological transition dates (PTDs; timing of green-up and dry-down), slopes during green-up and dry-down period, and seasonal amplitude, were extracted from time series of GPPmax and used to represent the seasonality of vegetation activity. The seasonal amplitude of GPPmax was higher for NT and NPT than CT, which was attributed to changes in structure and physiology induced by fertilization. PTDs were mainly driven by rainfall and exhibited no significant differences among treatments during the green-up period. Yet, both fertilized sites senesced earlier during the dry-down period (17-19days), which was more pronounced in the NT due to larger evapotranspiration and water usage. Fertilization also resulted in a faster increase in GPPmax during the green-up period and a sharper decline in GPPmax during the dry-down period, with less prominent decline response in NPT. Overall, we demonstrated seasonality of vegetation activity was altered after fertilization and the importance of nutrient-water interaction in such water-limited ecosystems. With the projected warming-drying trend, the positive effects of N fertilization induced by N deposition on GPPmax may be counteracted by an earlier and faster dry-down in particular in areas where the N:P ratio increases, with potential impact on the carbon cycle of water-limited ecosystems.
Highlights
Gross primary productivity (GPP), the most important component of the terrestrial carbon cycle (Beer et al, 2010; Ciais et al, 2014), varies greatly over space and time
In the N addition (NT), both structure and physiology contributed to changes in GPPmax, while in N and P addition (NPT) structure was the main driver
Precipitation determines the timing of green-up and the onset of photosynthetic activity (Figure 4)
Summary
Gross primary productivity (GPP), the most important component of the terrestrial carbon cycle (Beer et al, 2010; Ciais et al, 2014), varies greatly over space and time. Maximum gross primary productivity (GPPmax), representing the carbon sequestration potential of vegetation activity (i.e., photosynthetic capacity) and dominating the interannual variability of global net ecosystem exchange (NEE; Fu et al, 2019), in conjunction with extracted metrics from its time series, can be used to describe seasonal variation of GPP. Plants simultaneously balance uptake of multiple resources (i.e., carbon, nutrients, and water), and alter allocation to maximize acquisition of the most limiting resource (Bloom, Chapin, & Mooney, 1985; Chapin, Schulze, & Mooney, 1990) This means that both background ecosystem nutrient stoichiometry (e.g., N:P ratio) and the threefold increase in anthropogenic nitrogen deposition (Peñuelas, Sardans, Rivas-ubach, & Janssens, 2012; Reay, Dentener, Smith, Grace, & Feely, 2008) may alter seasonality of vegetation activity and phenology due to variations in nutrient availability (Piao et al, 2019). Understanding the links between nutrient availability and seasonal patterns of photosynthetic capacity (e.g., phenology of GPPmax) is crucial to understand temporal
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