Cattle exclusion induced dramatic changes in the plant community and modifications in nutrient cycling in grazed native grasslands of the Flooding Pampa (Argentina). The study was carried out to analyze the effect of grazing on the status and spatial variability of soil organic matter, nitrogen and phosphorus. Sampling was performed in the late summer and early spring. Geostatistical methods were used to study the spatial dependence of these soil properties. Organic carbon (OC) and total nitrogen (TN) showed spatial structure only in the ungrazed area with a similar range of dependence (39 m and 36 m respectively). The occurrence of litter in this area lead to a large and spatially homogeneous C input to the soil, which would be the key factor of the spatial structure of organic carbon and total nitrogen. Mineral nitrogen content [(NO3-)-N + (NHd+)-N] was higher in the ungrazed area on both sampling dates. The mineral N content showed a large short-range variahility (nugget variation) independent of grazing history. A significant decrease in the extractable P (Bray & Kurtz #l) in the grazed area was found. The extractable P exhibited spatial structure only in the ungrazed area. However, its spatial pattern was different from those of organic carbon and total nitrogen: the range of dependence was higher (57 m) and the spatial structure exhibited a great irregularity. The differences between C, N, and P variability were possibly related to their dynamics in the soil. No evidence of effects of animal excreta on nutrient content or spatial variability was found.

Cone serotiny refers to canopy seed storage in cones that remain alive, though closed, for several years. There are some Cupressus species which have only serotinous cones, or both serotinous and non-serotinous cones on the same tree. Scale separation and seed release occurs immediately after the cone ripens in nonserotinous cones, or else the cones remain closed and alive longer, generally up to 15-20 years. Germination tests were done with serotinous cone seeds from C. arizonica Greene, C. benthamii Endl., C. lusitanica Mill., and C. macrocarpa Hartw. Germination was below 50% for all species. The highest levels of germination were obtained from 2-to 7-year-old cones, depending on the species. Cupressus arizonica showed a regular decline in germination values, from 43.5% for seeds from 3-year-old cones to 7.5% for seeds from 7-year-old cones. Cupressus benthamii had a maximum germination value of seeds from 5-year-old cones. Cupressus macrocarpa had a maximum germination capacity of 20.5% for seeds from 6-year-old cones, and it showed 7.5% germination for 12-year-old seeds. Cupressus lusitanica reached 33.5% germination of seeds from 5-year-old cones. Serotinous cones from Cupressus species, usually not collected because it is assumed that their seeds are not viable, can be utilized in cone harvesting, increasing the yield of the cone-bearing trees.

No abstract