Interannual variability of net ecosystem carbon production and its climatic and biotic mechanism during 2005-2018 in a rain-fed maize ecosystem

The interannual variation (IAV) of net ecosystem carbon production (NEP) plays an important role in understanding the mechanisms of the carbon cycle in the agriculture ecosystem. In this study, the IAV of NEP, which were expressed as annual values and anomalies, and its climatic and biotic controls mechanism, were investigated based on an eddy covariance dataset of rain-fed spring maize during 2005–2018 in the northeast of China. The annual NEP was 270±115 g C m−2yr −1. Annual values and anomalies of NEP were positively correlated with that of precipitation (PPT), gross ecosystem production (GEP) and daily maximum NEP (NEPmax). Annual anomalies of NEP were dominantly and positively controlled by the soil water content (SWC) through GEP and the soil temperature (Ts) through RE. In comparison, annual anomalies of NEP were dominantly and negatively controlled by summer VPD through the NEPmax, positively adjusted by spring precipitation and the effective accumulative temperature through the beginning date (BDOY) of the affecting carbon uptake period (CUP), and by autumn precipitation and leaf area index through the end date (EDOY) of the affecting CUP. Residues restrained the carbon release at the beginning of the year, and accelerated the carbon release at the end of the year. Our results hightlight that NEP might be more sensitive to the change of water condition (such as PPT, SWC and VPD) induced by the climate changes.


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The interannual variation (IAV) of net ecosystem carbon production (NEP) plays an 24 important role in understanding the mechanisms of the carbon cycle, and it focuses on the analysis 25 of annual average values and anomalies [1][2][3][4]. Cultivated land occupies approximately 40% of the 26 terrestrial ecosystem and 30% is used for agriculture [5]. In the Northern Hemisphere, croplands 27 had positive contributions to the long term trend of land carbon uptake with 21%, and it played an 28 important role in the IAV of NEP in the terrestrial ecosystem [1]. Agricultural ecosystems may 29 fluctuate among a net carbon source, a sink, or neutral under the influences of climate change [6]. gross ecosystem productivity (GEP) and ecosystem respiration (RE) [9][10][11][12][13], or the integration of 36 carbon uptake or release peak and corresponding duration [1,[14][15][16], is crucial for identifying and 37 predicting the carbon cycle.

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Generally, it is necessary to explore the causes of variation in GEP and RE for understanding  152 where the terms on the right side of the equation are eddy flux (Term I), storage (Term II), horizontal advection (Term III), vertical advection (Term IV) (Table 1), and their annual 231 anomalies were estimated. The annual average NEP was 270±115 g C m −2 yr −1 (Table 1) annual anomalies of NEP were also negatively correlated with PAR and VPD (Table S2). In 235 addition, annual PAR and VPD had the significant increasing trends (Table 1).

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Annual anomalies of NEP were negatively dominated by VPD, and VPD was negatively adjusted 254 by PPT and positively adjusted by PAR (Fig. S3).

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3.2 Climatic and biotic controls of the IAV of NEP through GEP and RE

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Annual values and anomalies of NEP were positively and significantly correlated with GEP 257 but not with RE (Table S3). This result indicated that the climatic and biotic factors controlling 258 GEP were easier to lead to IAV of NEP, compared with the factors controlling RE. In addition,

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The SEM result showed that climatic and biotic controls through GEP and RE explained 88% 265 of annual anomalies of NEP (Fig. 3    The SEM analysis showed that 89% of annual anomalies of NEP were explained by climatic 301 and biotic controls through CUP and NEP max during the whole year (Fig. 4), and 91% during the 302 carbon uptake period (Fig. S4)

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Note that CUP was negatively controlled by BDOY and positively controlled by EDOY.

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BDOY was negatively correlated with spring precipitation and the effective accumulative 310 temperature. EDOY was positively correlated with LAI and autumn precipitation. NEP max had a 311 negative correlation with summer VPD. The relative contributions of CUP, NEP max and α for 312 annual anomalies of NEP during the uptake period were 9.7%, 84.1% and −3.1% (Table 4),

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respectively.   (Fig. 5a). Annual anomalies of the carbon release amount at the beginning of the     literature on different farmlands at the regional scale (Fig. 6), showed that NEP had a significant 379 positive correlation with GEP, but not with RE, and the ratio of RE and GEP was 0.78 (Table S6).

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In this study, annual values and anomalies of NEP had the significant positive correlation with

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GEP, but not with RE. Meanwhile, there was a significant positive correlation between RE and 382 GEP (Table S3) with a ratio of 0.75.

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The IAV of GEP was controlled by temperature at the regional scale and driven by moisture,   (Fig 4).

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Annual anomalies of CUP can be affected by temperature, precipitation, radiation, and LAI.

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The precipitation, and the EDOY was positively correlated with autumn precipitation and LAI (Fig. 4).