The conversion of two-thirds of New Zealand's native forests and grasslands to agriculture has followed trends in other developed nations, except that pastoral grazing rather than cropping dominates agriculture. The initial conversion of land to pasture decreased soil acidity and elevated N and P stocks, but caused little change in soil organic C stocks. However, less is known about C and nutrient stock changes during the last two decades under long-term pastoral management. We resampled 31 whole soil profiles in pastures spanning seven soil orders with a latitudinal range of 36-461S, which had originally been sampled 17-30 years ago. We measured total C, total N, and bulk density for each horizon (generally to 1 m) and also reanalyzed archived soil samples of the same horizons for C and N. On average, profiles had lost significant amounts of C (À2.1 kg C m À2 ) and N (À0.18 kg N m À2 ) since initial sampling. Assuming a continuous linear decline in organic matter between sampling dates, significant losses averaged 106 g C m À2 yr À1 (P 5 0.01) and 9.1 g N m À2 yr À1 (P 5 0.002). Removal of C through leaching and erosion appears too small to explain these losses, suggesting losses from respiration exceed the inputs of photosynthate in the soil profile. These results emphasize that resampling soil profiles provide a robust method for detecting soil C changes, and add credence to the suggestion that soil C losses may be occurring in some temperate soil profiles. Further work is required to determine whether these losses are continuing and how losses might be extrapolated across landscapes to determine the implications for New Zealand's national CO 2 emissions and the sustainability of the implied rates of soil N loss.

Grazing strategies promote changes in sward structure that can affect patterns of herbage accumulation and sward flexibility to management. This experiment evaluated sward structural characteristics and herbage accumulation of Mombaça guinea grass (Panicum maximum Jacq. cv. Mombaça) subjected to rotational stocking managements. Treatments involved grazing when swards reached either 95 or 100% interception of the incident light (LI), to post-grazing heights of 30 and 50 cm, and were allocated to 2,000 m 2 paddocks according to a randomised complete block design in a 2 × 2 factorial arrangement with four replications, from January 2001 to February 2002. Sward canopy changed from a vertical to a more horizontal orientation as the light interception area index and height increased from post to pre-grazing. Leaf canopy height pre-grazing was stable throughout the year and around 90 and 115 cm for the 95% and 100% LI treatments respectively, indicating a potential for development and use of target-based grazing management practices. Herbage mass pre and postgrazing was lower for the 95% than the 100% LI treatments, but with higher proportion of leaf and lower proportions of stem and dead material. Treatment effects on herbage accumulation were cumulative and became more evident as the experiment progressed, with up to 6 t ha -1 DM advantage to the 95/30 treatment. Although based on a single year experiment, grazing management should aim for low herbage mass during the transition period from winter to spring in order to allow fast recovery of swards and favour herbage accumulation during the following growing season. Key words: grazing management, leaf canopy height, leaf area index, light interception CARACTERÍSTICAS ESTRUTURAIS E ACÚMULO DE FORRAGEM EM PASTOS DE Panicum maximum cv. MOMBAÇA SUBMETIDOS A ESTRATÉGIAS DE PASTEJO ROTATIVORESUMO: Estratégias de pastejo promovem alterações na estrutura do dossel, afetando os padrões de acúmulo de forragem e flexibilidade ao manejo. O objetivo deste experimento foi avaliar as características estruturais do dossel e o acúmulo de forragem em pastos de capim-mombaça (Panicum maxium Jacq. cv. Mombaça) submetidos a estratégias de pastejo rotativo. Os tratamentos corresponderam a pastejos realizados quando os pastos atingiam 95 ou 100% de interceptação da luz incidente (IL) até alturas de resíduo de 30 e 50 cm, e foram alocados às unidades experimentais (piquetes de 2000 m 2 ) segundo um delineamento de blocos completos casualizados e arranjo fatorial 2 × 2, com quatro repetições, de janeiro de 2001 a fevereiro de 2002. O dossel passou de vertical para uma orientação mais horizontal à medida que o índice de área da folhagem e altura dos pastos aumentou durante a rebrota. A altura pré-pastejo do dossel mostrou-se estável durante o experimento e em torno de 90 e 115 cm para os tratamentos de 95 e 100% de IL, respectivamente, indicando potencial para o desenvolvimento e uso de práticas de manejo baseadas em metas de condição de pasto. A massa de forragem pré e pós-pastejo f...

Following clearance of the indigenous forest and conversion of the land to pasture early in the 20th century, gully erosion became a pervasive feature in the headwaters of the Waipaoa River basin, and was notably problematic in the 140-km 2 area now covered by the Mangatu Forest. In this area, before reforestation in 1961, gully erosion affected c. 4% of the terrain. After a 24-year exotic reforestation programme the area affected by gullies was reduced to 1.5%, but of the eight gullies larger than 10 ha in 1960 none had stabilized by 1988, although four had at least halved in size. Estimates that a gully will stabilize or increase in size under a range of conditions suggest that in the case of gullies <1 ha in area, formed in terrain underlain by Cretaceous rock, there is a >80% probability of stabilization after one forest rotation (c. 24 years). For gullies between 1 and 5 ha in area the probability of stabilization is c. 60%. Gullies of 5 ha have an even chance of stabilizing over the time frame of a single rotation. The key determinant is gully size and shape at the time of planting and, within this size range, these relationships were stronger for linear than for amphitheatre-shaped gullies. Between 1939 and 1988 sediment production from gullies in the portion (76%) of the Mangatu Forest underlain by Cretaceous-aged rock was c. 22 000 t km À2 yr À1 , and during the period of maximum sediment production they may have accounted for c. 17% of the Waipaoa River's average annual suspended sediment load. Reforestation reduced the contribution to c. 8% in the period between 1970 and 1988. However, the off-site (downstream) impact of sediment generated by the remaining 420 active gullies in the Waipaoa River catchment is significant, not least on the capacity of the scheme that protects high-value agricultural land on the Poverty Bay Flats from flooding. A targeted reforestation programme may be an alternative to raising the height of the existing artificial levees. It is estimated that additional exotic plantings totalling c. 15 400 ha (c. 7% of the Waipaoa River basin area) would produce a >64% reduction in sediment production from gullies on pastoral hillslopes within one forest rotation (c. 24 yr).

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