Nonadditive and Legacy Effects of Spring and Autumn Warming on Soil Respiration in an Old-Field Grassland

Asymmetric pre-growing season warming may jeopardize seed reproduction of the sand-stabilizing shrub Caragana microphylla

Our current understanding of the processes and mechanisms by which seasonal asymmetric warming affects seed reproduction in semiarid regions, which are essential in preserving the stability of both vegetation ecosystem structure and function, remains poorly understood. Here, we conducted a field warming experiment, including pre-growing season warming (W1), in-growing season warming (W2), and combined pre- and in-growing season warming (W3) treatments, to investigate the seed reproductive strategy of Caragana microphylla, an important sand-stabilizing shrub, from the perspective of reproductive phenology, reproductive effort, and reproductive success. Results show that the warming treatments advanced the initial stages of reproductive phenology, prolonged its duration, and decreased its synchrony (magnitude = W3 > W2 > W1). Additionally, flowering phenology was more sensitive to warming than podding phenology. The W1 treatment inclined seed reproduction towards the conservative strategy with low reproductive effort and success. The W3 treatment tended to increase seed reproductive effort and success. While the W2 treatment did not affect reproductive success, it did increase reproductive effort. Changes in reproductive phenology explained 20 % of the variation in reproductive effort and 38 % of the variation in reproductive success. However, these changes also directly hindered reproductive success (direct effect = -0.57) while indirectly promoting reproductive success (indirect effect = 0.27) by increasing reproductive efforts. Our results reveal that the seasonal asymmetry of warming altered the seed reproduction strategy of sand-stabilizing shrubs, with warmer winters and springs before the growing season decreasing seed fecundity.

Effect of Precipitation Variation on Soil Respiration in Rain-Fed Winter Wheat Systems on the Loess Plateau, China

Global climate change has aggravated the hydrological cycle by changing both the amount and distribution of precipitation, and this is especially notable in the semiarid Loess Plateau. How these precipitation variations have affected soil carbon (C) emission by the agroecosystems is still unclear. Here, to evaluate the effects of precipitation variation on soil respiration (Rs), a field experiment (from 2019 to 2020) was conducted with 3 levels of manipulation, including ambient precipitation (CK), 30% decreased precipitation (P−30), and 30% increased precipitation (P+30) in rain-fed winter wheat (Triticum aestivum L.) agroecosystems on the Loess Plateau, China. The results showed that the average Rs in P−30 treatment was significantly higher than those in the CK and P+30 treatments (p < 0.05), and the cumulative CO2 emissions were 406.37, 372.58 and 383.59 g C m−2, respectively. Seasonal responses of Rs to the soil volumetric moisture content (VWC) were affected by the different precipitation treatments. Rs was quadratically correlated with the VWC in the CK and P+30 treatments, and the threshold of the optimal VWC for Rs was approximately 16.06−17.07%. However, Rs was a piecewise linear function of the VWC in the P−30 treatment. The synergism of soil temperature (Ts) and VWC can better explain the variation in soil respiration in the CK and P−30 treatments. However, an increase in precipitation led to the decoupling of the Rs responses to Ts. The temperature sensitivity of respiration (Q10) varied with precipitation variation. Q10 was positive correlated with seasonal Ts in the CK and P+30 treatments, but exhibited a negative polynomial correlation with seasonal Ts in the P−30 treatment. Rs also exhibited diurnal clockwise hysteresis loops with Ts in the three precipitation treatments, and the seasonal dynamics of the diurnal lag time were significantly negatively correlated with the VWC. Our study highlighted that understanding the synergistic and decoupled responses of Rs and Q10 to Ts and VWC and the threshold of the change in response to the VWC under precipitation variation scenarios can benefit the prediction of future C balances in agroecosystems in semiarid regions under climate change.