In the aerobic rice system, adapted rice cultivars are grown in non-flooded moist soil. Aerobic rice may be suitable for double cropping with winter wheat in the Huai River Basin, northern China plain. Field experiments in 2005 and 2006 were conducted to study the response of aerobic rice and winter wheat to sequential rates of nitrogen (N), phosphorus (P) and potassium (K) in aerobic ricewinter wheat (AR-WW) and winter wheat-aerobic rice (WW-AR) cropping sequences. Fertilizer treatments consisted of a complete NPK dose, a PK dose (N omission), a NK dose (P omission), a NP dose (K omission), and a control with no fertilizer input. Grain yields of crops with a complete NPK dose ranged from 3.7 to 3.8 t ha -1 and from 6.6 to 7.1 for aerobic rice' and 'winter wheat', respectively. N omissions caused yield reductions ranging from 0.5 to 0.8 t ha -1 and from 1.6 to 4.3 t ha -1 for rice and wheat, respectively. A single omission of P or K did not reduce rice and wheat yields, but a cumulative omission of P or K in a double cropping system significantly reduced wheat yields by 1.2-1.6 t ha -1 . N, P and K uptake of both crops were significantly influenced by fertilizer applications and indigenous soil nutrient supply. Nutrient omissions in a preceding crop reduced plant N and K contents and uptake additionally to direct effects of the fertilizer treatments in wheat, but not in rice. Apparent nutrient recoveries (ANR) differed strongly between 'aerobic rice' and 'winter wheat'; in rice: for N it ranged from 0.30 to 0.32, for P from 0.01 to 0.06, and for K from 0.03 to 0.19 and in wheat: for N from 0.49 to 0.71, for P from 0.09 to 0.15, and for K from 0.26 to 0.31. Further improvements of crop productivity as well as nutrient-use efficiencies, should be brought about by developing cropping systems, by an appropriate choice of adapted cultivars, by a site-and timespecific fertilizer management and by eliminating other yield-limiting factors. It is concluded that nutrient recommendations should not be based on the yield response of single crops only, but also on the after-effects on nutrient availability for succeeding crops. A whole cropping system approach is needed.
Efficient subsampling designs reduce forest inventory costs by focusing sampling efforts on more variable forest attributes. Sector subsampling is an efficient and accurate alternative to big basal area factor (big BAF) sampling to estimate the mean basal area to biomass ratio. In this study, we apply sector subsampling of spherical images to estimate aboveground biomass and compare our image-based estimates with field data collected from three early spacing trials on western Newfoundland Island in eastern Canada. The results show that sector subsampling of spherical images produced increased sampling errors of 0.3–3.4 per cent with only about 60 trees measured across 30 spherical images compared with about 4000 trees measured in the field. Photo-derived basal area was underestimated because of occluded trees; however, we implemented an additional level of subsampling, collecting field-based basal area counts, to correct for bias due to occluded trees. We applied Bruce’s formula for standard error estimation to our three-level hierarchical subsampling scheme and showed that Bruce’s formula is generalizable to any dimension of hierarchical subsampling. Spherical images are easily and quickly captured in the field using a consumer-grade 360° camera and sector subsampling, including all individual tree measurements, were obtained using a custom-developed python software package. The system is an efficient and accurate photo-based alternative to field-based big BAF subsampling.
Big basal area factor (big BAF) sampling is a widely used subsampling method to select measure-trees. Several studies have shown big BAF sampling to be an efficient sampling scheme. In this study, we use sector sampling (Smith et al. 2008, For. Sci. 54: 67–76) as an alternative subsample selection method. Based on some simulated mapped stands derived from three balsam fir (Abies balsamea (L.) Mill.) spacing trials in western Newfoundland, we show that sector subsampling is comparable to big BAF sampling in terms of estimated mean basal area ratios and their associated standard errors. Differences between big BAF sampling and sector sampling methods showed less than 1% difference across the three sites. As with big BAF sampling, changes in sample intensity had no significant (p < 0.05) effects on the accuracy of estimating mean biomass to basal area ratios and the resulting estimated mean biomasses per unit area.
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