Background Farmland accounts for a relatively large fraction of the world’s vegetation cover, and the quantification of carbon fluxes over farmland is critical for understanding regional carbon budgets. The carbon cycle of farmland ecosystems has become a focus of global research in the field of carbon dynamics and cycling. The objectives of this study are to monitor the temporal variation in the net ecosystem exchange (NEE) and soil respiration in a spring maize ( Zea mays L.) farmland ecosystem of the southern Loess Plateau of China. Methods A fully automated temperature-controlled flux chamber system was adopted in this study. The system contained nine chambers for CO 2 flux measurements, and three treatments were conducted: with and without maize plants in the chamber, as well as a bare field. Observations were conducted from June to September 2011. This time period covers the seedling, jointing, heading, grain filling, and ripening stages of spring maize. Other factors, such as air temperature (Ta), soil temperature (Ts), soil water content (SWC), photosynthetically active radiation (PAR), and precipitation (P), were simultaneously monitored. Results There was observed diurnal variation in the NEE of the maize ecosystem (NEE-maize). A short “noon break” occurred when the PAR intensity was at its maximum, while soil respiration rates had curves with a single peak. During the overall maize growth season, the total NEE-maize was –68.61 g C m −2 , and the soil respiration from the maize field (SR-maize) and bare field (SR-bare field) were 245.69 g C m −2 and 114.08 g C m −2 , respectively. The temperature sensitivity of soil respiration in the maize field exceeded that in the bare field. Significant negative correlations were found between the NEE, PAR, and temperature (all p -values < 0.01), with both Ta and PAR being the primary factors that affected the CO 2 fluxes, collectively contributing 61.7%, 37.2%, and 56.8% to the NEE-maize, SR-maize, and SR-bare field, respectively. It was therefore concluded that both meteorological factors and farming practices have an important impact on the carbon balance process in corn farmland ecosystems. However, it is necessary to conduct long-term observational studies, in order to get a better understanding of the driving mechanism.
Abstract Crago et al. published two papers in Water Resources Research in 2016 and 2018, respectively, which discussed recent generalized complementary evaporation relationships; they recommended that the lower boundary condition of the Generalized Nonlinear Advection‐Aridity (GNAA) model proposed by Brutsaert (2015, https://doi.org/10.1002/2015WR017720 ) should be modified to y = 0 at x = x min , where x min ∈[0,1]. First, it is shown herein that the rescaled complementary relationship (CR) y = X, proposed by Crago et al., does not advance the analysis forward, but that instead the rescaling is tantamount to a reversion to an earlier linear and more traditional asymmetric complementary model, in which the parameter b is a function of x min . In fact, at x min = 0, y = X the rescaled formulation becomes physically unrealistic. The second part of this comment deals with the extended quartic form of GNAA model. The nature of the adjustable parameter c in this extended formulation is further explored, and it is shown that while its lower limit is −1, calibration with actual data can result in values in excess of 2. Such calibration allows then the estimation of x min as a function of c, that is, x min = f (c). This way, in addition to satisfying the original four boundary conditions, the extended GNAA model also satisfies the boundary condition of y = 0 at x = x min . The physical variables affecting the nature of the parameter c and its relationship with E po /E pmax will require further investigation.
The influence of surface mulching on soil aggregation and associated carbon (C) and nitrogen (N) varies by mulching materials and crop types. The 6 yr effect of straw mulching (SM), plastic film mulching (PM), and no mulching (CK) on soil aggregation and associated C and N concentrations at 0–20 and 20–40 cm soil layers were studied under dryland winter wheat (Triticum aestivum L.) and spring corn (Zea mays L.) in the Loess Plateau of China. Regardless of crop types, aggregate proportion was greater in macroaggregates (2.00–10.00 mm), but lower in microaggregates (<0.50 mm) with mulching than without in both soil layers. The mean weight diameter of aggregates was greater with SM and PM than CK. Compared with CK and PM, SM increased soil organic C (SOC) and total N (STN) concentrations in both macroaggregates and bulk soil at 0–20 cm. Aggregate proportion and soil C and N concentrations at both depths were more pronounced in winter wheat than spring corn. The recovery rates of bulk soil SOC and STN in aggregates varied from 94% to 107%. Straw and plastic film mulching enhanced soil aggregation compared with no mulching. Straw mulching was more effective in increasing SOC and STN concentrations at the surface layer in dryland winter wheat and spring corn.
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