Response of biocrusts' photosynthesis to environmental factors:a possible explanation of the spatial distribution of biocrusts in the Hilly Loess Plateau region of China

Biological soil crusts(biocrusts) are a ubiquitous living cover in grasslands that were rehabilitated during the "Grain for Green" eco-project implemented in the Loess Plateau region of China.In mature vascular plant communities,biocrusts often cover up to 60%-70% the soil surface,whereas in newly restored areas biocrust cover can be up to 90%. Despite being a substantial source of soil organic material,little research has been done on photosynthetic and respiration activities and on how photosynthetic and respiration rates in different successional stages of biocrusts respond to varying environmental factors including light intensity,water content and temperature.To further understand this,we collected samples in Hilly Loess Plateau region from a 3 year-old rehabilitated site representing an early successional biocrust community dominated by cyanobacteria,and from a 13-year rehabilitated site,representing a well developed late successional biocrust community that was dominated by the moss Barbula vinealis.Using a semi-open countertop CO2 gas exchange under controlled laboratory conditions,we measured the response of photosynthesis and dark respiration of four replicate samples of each kind of biocrust.We subjected them to five different water contents that ranged from 20% to 100% field water holding capacity in 20% increments,15 different light intensities from 0 to 1800 μmol·m-2·s-1 PAR(photosyntheticall active radiation),and six temperatures that increased from 5 to 30℃ in 5℃ increments.Results showed that(1) photosynthesis and dark respiration activities in biocrusts from this region were controlled mainly by the composition of the biocrust community.Photosynthesis and dark respiration rates in moss-dominated biocrusts were significantly higher than those in cyanobacterialy-dominated biocrusts when measured under similar temperature and light intensity with fully activated biocrusts samples. (2) Water was a critical environmental factor in determining photosynthetic rates of biocrusts,in that gas exchange was possible only after a certain moisture level was reached.The optimal moisture for photosynthesis(both net and gross photosynthesis) in moss and cyanobacteria dominated biocrusts from the study region were 80%-100% and 40%-80% field water capacity,respectively. (3) Both moss and cyanobacteria dominated biocrusts showed a low light compensation point that was below 10μmol·m-2·s-1 PAR.The light saturation point for moss-and cyanobacterialy-dominated biocrusts were 1000 and 800μmol·m-2·s-1 PAR,respectively.Light inhabitation for photosynthesis was not found in either kind of biocrust. (4) Temperature showed a significant influence on photosynthesis and dark respiration in biocrusts.The optimal temperature for photosynthesis(both net and gross photosynthesis) in moss and cyanobacteria dominated biocrusts from Hilly Loess Plateau region were 20-25℃ and 25-30℃,respectively. (5)Response of biocrusts' net photosynthesis to environmental factors likely explains the observed spatial distribution pattern of biocrusts in the research region.We have observed cyanobacteria crusts generally dominate the sunny,open habitats in these grasslands,whereas moss-dominated crusts are found in shadier habitats.Accordingly,our results show cyanobacterialy-dominated crusts have higher carbon gain at lower moisture and higher temperatures(similar to those found in the sunny open habitats),relative to the moss-dominated crusts,whose carbon gain was greater under cooler and moisture conditions(as is found in shadier habitats).