Effects of organic mulching on soil aggregate stability and aggregate binding agents in an urban forest in Beijing, China

Urban forest soil is often disturbed by rapid urbanization. Organic mulching is effective for improving soil quality and aggregate stability. This study evaluated how soil binding agents changed aggregate stability through organic mulching in urban forest soils. Three treatments were applied in Jiufeng National Forest Park, Beijing: (1) no organic mulch (control);(2) wood chips alone (5 cm thickness);and, (3) wood chips + wood compost (This mulch was divided into two layers, the upper layer of wood chips (2.5 cm), the lower layer wood compost (2.5 cm)). Soil samples were collected from the surface 10- cm soil layer and fraction into four aggregates. Glomalin-related soil protein and soil organic carbon were measured in bulk soil and the four aggregates. The results show that wood chips + wood compost increased the proportion of large and small macroaggregates, mean weight diameter and geometric mean diameter. The total and easily extractable glomalin-related soil protein were higher in the wood chips + wood compost. However, soil organic carbon was lower in the wood chips alone application compared to the controls and wood chips + wood compost. Easily extractable / total glomalin-related soil protein and glomalin-related soil protein / soil organic carbon ratios of wood chips alone and wood chips + wood compost had increased trend compared to the controls but did not reach significant levels (p > 0.05). Mean weight diameter and geometric mean diameter correlated positively with total and easily extractable glomalin-related soil protein but were not positively correlated with soil organic carbon, the ratios of easily extractable and total glomalin-related soil protein, and the ratios of glomalin-related soil protein and soil organic carbon. Redundancy analysis revealed that total glomalin-related soil protein was the most important driver for soil aggregate stability, especially the total glomalin-related soil protein of small macroaggregates. The results suggest that wood chips + wood compost enhanced soil aggregate stability through the increase of glomalin-related soil protein. Wood chips alone cannot enhance soil aggregate stability in urban forests in the short term.

Organic mulching promotes soil organic carbon accumulation to deep soil layer in an urban plantation forest

Background:Soil organic carbon(SOC)is important for soil quality and fertility in forest ecosystems.Labile SOC fractions are sensitive to environmental changes,which reflect the impact of short-term internal and external management measures on the soil carbon pool.Organic mulching(OM)alters the soil environment and promotes plant growth.However,little is known about the responses of SOC fractions in rhizosphere or bulk soil to OM in urban forests and its correlation with carbon composition in plants.Methods:A one-year field experiment with four treatments(OM at 0,5,10,and 20 cm thicknesses)was conducted in a 15-year-old Ligustrum lucidum plantation.Changes in the SOC fractions in the rhizosphere and bulk soil;the carbon content in the plant fine roots,leaves,and organic mulch;and several soil physicochemical properties were measured.The relationships between SOC fractions and the measured variables were analysed.Results:The OM treatments had no significant effect on the SOC fractions,except for the dissolved organic carbon(DOC).OM promoted the movement of SOC to deeper soil because of the increased carbon content in fine roots of subsoil.There were significant correlations between DOC and microbial biomass carbon and SOC and easily oxidised organic carbon.The OM had a greater effect on organic carbon fractions in the bulk soil than in the rhizosphere.The thinnest(5 cm)mulching layers showed the most rapid carbon decomposition over time.The time after OM had the greatest effect on the SOC fractions,followed by soil layer.Conclusions:The frequent addition of small amounts of organic mulch increased SOC accumulation in the present study.OM is a potential management model to enhance soil organic matter storage for maintaining urban forest productivity.

Response of nitrogen fractions in the rhizosphere and bulk soil to organic mulching in an urban forest plantation

Nitrogen is an essential component in forest ecosystem nutrient cycling.Nitrogen fractions,such as dissolved nitrogen,ammonium,nitrate,and microbial biomass nitrogen,are sensitive indicators of soil nitrogen pools which affect soil fertility and nutrient cycling.However,the responses of nitrogen fractions in forest soils to organic mulching are less well understood.The rhizosphere is an important micro-region that must be considered to better understand element cycling between plants and the soil.A field investigation was carried out on the effect of mulching soil in a 15-year-old Ligustrum lucidum urban plantation.Changes in total nitrogen and nitrogen fractions in rhizosphere and bulk soil in the topsoil(upper 20 cm)and in the subsoil(20-40 cm)were evaluated following different levels of mulching,in addition to nitrogen contents in fine roots,leaves,and organic mulch.The relationships between nitrogen fractions and other measured variables were analysed.Organic mulching had no significant effect on most nitrogen fractions except for the rhizosphere microbial biomass nitrogen(MBN),and the thinnest(5 cm)mulching layer showed greater effects than other treatments.Rhizosphere MBN was more sensitive to mulching compared to bulk soil,and was more affected by soil environmental changes.Season and soil depth had more pronounced effects on nitrogen fractions than mulching.Total nitrogen and dissolved nitrogen were correlated to soil phosphorus,whereas other nitrogen fractions were strongly affected by soil physical properties(temperature,water content,bulk density).Mulching also decreased leaf nitrogen content,which was more related to soil nitrogen fractions(except for MBN)than nitrogen contents in either fine roots or organic mulch.Frequent applications of small quantities of organic mulch contribute to nitrogen transformation and utilization in urban forests.

Optimization of Organic Mulches Thickness Improves Soil Moisture Retention under Controlled Conditions

Organic mulch can improve the moisture,chemical composition,dust,and dust suppression of soil,and beautify the environment.In view of the rapid evaporation rate and serious loss of soil water in tropical areas,this paper explored the effect of organic mulch materials with different thickness on the increase of soil water retention rate and the improvement of soil water loss caused by evaporation.Rubberwood sawdust(RWS),rubberwood bark(RWB),coconut fiber(CF),and Mulch(MC)were selected as the mulching materials.Field experiment and laboratory experiment were performed,and soil-moisture content and temperature were continuously monitored.However,from the daily measurement of water content at constant conditions(29℃±0.2℃,74%±1%air RH)in the laboratory experiment,the results of variance analysis(ANOVA)showed that there was no significant difference between the soil-water content of covered samples and bare soil(P>0.05).In the field experiments,the analysis of variance indicated significant differences in the soil-moisture content owing to the effect of the covering material(P<0.01).Mulching increased the soil-moisture content with smaller fluctuations in the deep soil compared with bare soil.The most stable soil-moisture content were achieved by RWS,RWB,CF,and MC,with thicknesses of 5,3,7,and 5 cm,respectively,compared with bare soil,and the average water contents of the 0-40 cm soil layer was 0.58%,0.01%,0.82%,and 0.93%,respectively.Vertically,the intensity of the change in soil moisture decreased gradually with increasing depth,and was more stable than that of bare soil and other treatments.Among them,the difference in water content between the adjacent gradient soil layers(the soil layers are graded every 10 cm in depth)M_(3-7)(0.011±0.004)was the smallest.It can be concluded that CF mulching materials with a thickness of 7 cm would be preferable when selecting mulching materials for controlling soil moisture in tropical cities.