Increased Litter Greatly Enhancing Soil Respiration in Betula platyphylla Forests of Permafrost Region, Northeast China

The change of litter input can affect soil respiration (Rs) by influencing the availability of soil organic carbon and nutrients, regulating soil microenvironments, thus resulting in a profound influence on soil carbon cycle of the forest ecosystem. We conducted an aboveground litterfall manipulation experiment in different-aged Betula platyphylla forests (25-, 40- and 61-year-old) of the permafrost region, located in the northeast of China, during May to October in 2018, with each stand treated with doubling litter (litter addition, DL), litter exclusion (no-litter, NL) and control litter (CK). Our results indicated that Rs decreased under NL treatment compared with CK treatment. The effect size lessened with the increase in the stand age; the greatest reduction was found for young Betula platyphylla forest (24.46% for 25-year-old stand) and tended to stabilize with the growth of forest with the reduction of 15.65% and 15.23% for 40-and 61- year-old stands, respectively. Meanwhile, under DL treatment, Rs increased by 27.38%, 23.83% and 23.58% on 25-, 40- and 61-year-old stands, respectively. Our results also showed that the increase caused by DL treatment was larger than the reduction caused by NL treatment, leading to a priming effect, especially on 40- and 61-year-old stands. The change in litter input was the principal factor affecting the change of Rs under litter manipulation. The soil temperature was also a main factor affecting the contribution rate of litter to Rs of different-aged stands, which had a significant positive exponential correlation with Rs. This suggests that there is a significant relationship between litter and Rs, which consequently influences the soil carbon cycle in Betula platyphylla forests of the permafrost region, Northeast China. Our finding indicated the increased litter enhanced the Rs in Betula platyphylla forest, which may consequently increase the carbon emission in a warming climate in the future. It is of great importance for future forest management in the permafrost region, Northeast China.