Bush lupin Lupinus arboreus (native to western USA) may reduce native plant diversity through nitrogen (N) enrichment of formerly native-dominated habitats. Carbon addition by incorporation of sawdust into soil has been suggested as a possible technique to reduce plant-available N in nutrient-enriched grasslands in order to try and restore native herb and forb communities. This method was tested in an area periodically dominated by lupin.at Tom's Point (a private nature preserve), California, southwest USA (38°13′N, 122°57′W).
In summer 1998, sawdust (1.2 kg/m) was added to 70 (1.5 x 1.5 m) plots (pre-prepared by removing vegetation and glyphosate sprayed) containing various combinations of three native perennial bunchgrasses (bent grass Agrostis idahoensis bent grass, red fescue Festuca rubra and purple needlegrass Nassella pulchra), non-native perennial grasses (tall fescue Festuca arundinacea, Yorkshire-fog Holcus lanatus and harding grass Phalaris aquatica), and non-native annual grasses (slender oat Avena barbata, ripgut brome Bromus diandrus and rat’s-tail fescue Vulpia myuros).
Aboveground biomass of grasses was sampled in 1999 and 2000. Measurements of basal diameter, height and number of flowering culms of perennial grasses were made. Soils samples were periodically taken and analysed for extractable ammonium and nitrate, net N mineralization and microbial biomass N.
Sawdust addition resulted in higher soil microbial biomass N, lower rates of net N mineralization and nitrification, and higher concentrations of extractable soil ammonium.
In1999, sawdust addition decreased suppression of N. pulchra seedlings (and to a lesser extent F. rubra) by non-native annuals. However, growth of non-native grasses was not reduced in sawdust-addition plots. In 2000, native perennial grasses that survived the first year of competition with annual grasses reduced aboveground productivity of the annual grasses in non-sawdust addition plots just as well as those with sawdust added.
These results suggest that sawdust addition afforded no significant benefit to the restoration of these three native grasses at this site over the 2-year study period.
If using or referring to this published study, please read and quote the original paper, this can be viewed at: http://onlinelibrary.wiley.com/doi/10.1111/j.1061-2971.2004.00299.x/full