Background Research on the biomass root-shoot ratio of shelterbelts is conducive to understand the gravity pressure of underground root biomass and the spatial distribution of aboveground and underground biomass at different ages, so as to improve the structural stability of trees and ensure the sustainable development of ecological service function of shelterbelts. In order to explore the biomass accumulation process and the change trend of root-shoot ratio of farmland shelterbelts in oasis, this study was carried out.

Methods This study took the 2 rows farm-canal-forest-road type farmland shelterbelt in the Yellow River irrigation oasis of Dengkou in Ulanbuh desert as the research object. Space was used to replace time, and biomass of different diameter classes in the same space was used to replace biomass of different age diameter classes. Because the forest belt was closely connected with the canal, road farmland, and the investigation could not destroy the infrastructure, the fixed quadrat investigation method was adopted for the biomass of lateral roots.

Results 1) The aboveground and underground biomass of young forest in two-row canal shelterbelts were 15.212 kg and 6.539 kg, respectively, and the aboveground and underground biomass of half-mature, near-mature and mature forests increased by 8.77 times and 4.89 times, 2.82 times and 2.59 times, 1.65 times and 1.53 times, respectively. Seven biomass models were obtained. 2) The root-shoot ratio of biomass was in the order of young forest > half-mature forest > near-mature forest > mature forest, the average value was 24.35% and that for young forest was 43.67%. The root-shoot ratio of biomass of half-mature forest, near-mature forest and mature forest decreased by 1.79 times, 1.10 times and 1.08 times, respectively, and one biomass root-shoot ratio model was acquired. 3) The average density of biomass of trunk was 393.81 kg/m³, and that of young forest was 375.02 kg/m³; half-mature forest decreased by 3.09%, and near-mature and mature forest increased by 3.37% and 22.38%, respectively. 4) The biomass density of main roots of young forest was 404.20 kg/m³. The biomass density of half-mature forest and near-mature forests decreased by 4.14% and 0.48% respectively, while that of mature forest increased by 22.38%. The root dry density ratios of young, half-mature, near-mature and mature forest were 1.08, 1.07, 1.03 and 0.79, respectively.

Conclusions Under normal circumstances, both aboveground and underground biomass show an increasing trend with the increase of diameter at breast height (DBH), and the growth rate of aboveground biomass is much larger than that of underground biomass, the difference of biomass increases and the gap of growth rate decreases. With the increase of DBH, the root-shoot ratio of biomass tends a decreasing, and the decrease range is reduced. The biomass density of trunk firstly increases and later decreases with the increase of DBH, which is positively correlated with DBH. The biomass density of main roots decreases with DBH, which is negatively correlated with DBH.