Macro-element pools and fluxes in tall-tussock (Chionochloa) grasslands, Canterbury, New Zealand

Abstract

Above-ground element pools in stands dominated by Chionochloa rigida and C. macra range from: N, 143 and 64.4 kg/ha respectively; P, 24.4 and 9.5 kg/ha; to Na, 4.0 and 1.5 kg/ha. The proportion of elements in the dead compartment range from 76% of Ca to 35% of K in the C. rigida stand, and from 77% to 23% of these elements in the C. macra stand. Below-ground element pools range from: N, 153.6 and 100.0 kg/ha; P, 19.0 and 15.8 kg/ha; to Na, 10.4 and 7.3 kg/ha for the two stands. Total elements are in the order K>N>Ca>P>Mg>S>Na for C. rigida, and K>N>P>Ca>S>Mg>Na for C. macra. Sea-sonal differences between shoot weights plus elements lost via litter fall were used to calculate element uptake, e.g., P, 7.1 kg/ha/yr in the C. rigida stand and 3.7kg/ha/yr in the C. macra stand. Sheaths and blades analysed separately show K, Na, and P in younger leaves to be higher in sheaths than in blades. Ca and Mg increase from youngest to oldest blades. These patterns contrast with those for some other grasses. Seasonal variation in element concentrations suggests that C. macra and C. rigida may store elements in sheaths over winter. This is discussed in relation to patterns of leaf senescence. Chionochloa leaf litter placed out in mesh bags showed approximately negative-exponential loss rates over 3.3 years, winter losses were negligible. Estimated turnover times for C. rigida ranged from 6.7 years derived from litter fall to 21 years for 95% loss-times of sheaths in litter bags. Decomposition is slower at the higher-altitude C. macra site. Sheaths decompose less rapidly than blades. Decomposition is more rapid on the ground than in tussock crowns. Relative turnover times of individual elements differ between sites, partly it is inferred, because of greater element sorption at the C. rigida site. The effects of blades becoming detached from the sheaths (a characteristic of C. rigida but not C. macra) are discussed in relation to leaching of elements from standing dead material, and nutrient cycling.