[Characteristics of CO2 Flux in a Mature Apple (Malus demestica) Orchard Ecosystem on the Loess Plateau]

Abstract

Apples (Malus demestica) in the Loess Plateau region are grown in the largest apple orchards in the world and China, playing an important role in the improvement of the ecological environment. However, there is little research on the scale of the ecological system of the apple orchard in the Loess Plateau region. In this study, the CO₂ flux of a mature apple orchard in the Loess Plateau region was observed using an eddy covariance technique in the Shannxi Province. Based on the observation data sets observed from January 2016 to December 2016, a quantitative analysis of the apple orchard net ecosystem exchange (NEE), ecosystem respiration (R_eco), total ecosystem primary productivity (GPP) changes at different time scales, changes in main meteorological factors, the effects of soil temperature (T_s) and air temperature (T_a) at different levels, and PAR on NEE were discussed. The results showed that during the study period, the apple orchard ecosystem NEE monthly totals were positive (as a carbon source) in the non-growing season in December, January, February and March, were negative (a carbon sink) in the growing season from April to November, and functioned as a strong carbon sink year round. The maximum peak NEE (absolute value) monthly average daily change appeared in August[-17.08 μmol ·(m² ·s)^-1], and the smallest peak appeared in November[-4.47 μmol ·(m² ·s)^-1] during the growing season. The NEE monthly average daily change value during the non-growing season is very weak, though the change is not obvious. GPP, R_eco, and NEE maximum daily total values were 11.12, 5.04, and -7.34 g ·(m² ·d)^-1, respectively. GPP, R_eco, and NEE maximum monthly total values were 238.97, 105.38, and -144.44 g ·(m² ·month)^-1, respectively, as GPP and NEE maintained high cumulative values that were relatively stable from May through August. The annual GPP, R_eco, and NEE were 1223.2, 525.2, and -698.0 g ·(m² ·a)^-1. The observations show that the mature apple orchard ecosystem in the Chinese Loess Plateau has a relatively high carbon sequestration capacity. Nighttime ecosystem respiration R_eco.n was positively correlated with the soil temperature and air temperature at different levels, and the correlation coefficients were T_{s-5 cm} > T_{s-10 cm} > T_{a-4 m} > T_{a-8 m}, The photosynthetic active radiation (PAR) can explain more than 80% of the daytime NEE changes.

Keywords: CO2 flux; Loess Plateau; apple orchard; carbon sink; eddy covariance.