Cropping intensity and rainfall effects on upland rice yields in northern Laos

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1 Plant Soil (2006) 284: DOI /s RESEARCH ARTICLE Cropping intensity and rainfall effects on upland rice yields in northern Laos K. Saito Æ B. Linquist Æ B. Keobualapha Æ K. Phanthaboon Æ T. Shiraiwa Æ T. Horie Received: 27 March 2005 / Accepted: 23 February 2006 Ó Springer Science+Business Media B.V Abstract In northern Laos, upland rice is grown as a subsistence crop under rainfed conditions with no fertilizer inputs. It has traditionally been grown under slash-and-burn systems with long fallows, which restore soil fertility and reduce insect and weed pressure. However, increasing population density and government policies aimed at reducing the area under slash-and-burn have reduced fallows to as little as two or three years between rice crops. In this paper the impact of intensifying upland rice cultivation and rainfall on upland rice productivity was evaluated Section Editor: R. W. Bell. K. Saito (&) Æ T. Shiraiwa Æ T. Horie Graduate school of Agriculture, Kyoto University, Kyoto, Japan k.saito@cgiar.org B. Linquist Department of Plant Sciences, University of California, Davis 95616, USA B. Keobualapha Provincial Agriculture and Forestry Office, Luang Prabang, Lao PDR K. Phanthaboon Northern Regional Agriculture and Forestry Research Center, Luang Prabang, Lao PDR Present Address: K. Saito Africa Rice Center (WARDA), 01 BP 2031 Cotonou, Benin using yield and rainfall data from Luang Prabang province from 1992 to In addition, an experiment was conducted in 2004 to evaluate the effect of upland rice cropping intensification on soil nitrogen (N) and phosphate (P) availability and root pests (Tetraneura nigriabdominalis-root aphids and Meloidogyne graminicola Golden & Birchfield-nematodes).Rice yields were associated with total rainfall from June through August, corresponding to mid-tillering through flowering growth stages of upland rice. Increased cropping intensity resulted in a significant reduction of upland rice yields with higher rice yields being associated with longer fallows. Furthermore, when rice was annually cropped in the same field without fallows, rice yields rapidly declined. A study conducted in 2004 indicated that increasing cropping intensity reduced the soil N and P availability and increased root aphid infection of rice. The long-term productivity of upland rice can not be sustained with increased cropping intensity using the current management practices. Therefore, improved crop and resource management technologies are necessary for sustainable production. Keywords Cropping intensity Æ Laos Æ Nitrogen Æ Rainfall Æ Root aphids Æ Slash-and-burn Æ Upland rice Introduction Rice is the most important crop in Laos and about 70% of the total calorie supply in diets comes from

2 176 Plant Soil (2006) 284: rice (Maclean et al. 2002). In the mountainous region of northern Laos the upland rice ecosystem accounts for about half of the total rice area (National Statistical Center 2004). Upland rice is typically grown under slash-and-burn systems and farmers have relied on extended fallows to restore soil fertility and to reduce problems from insects and weeds as is typical of slash-and burn systems elsewhere (Nye and Greenland 1960). In Laos, the traditional cropping cycle has been a single rice crop followed by fallow periods of 10 or more years (Roder et al. 1997a; Roder 2001). It is generally agreed that slash-andburn systems are sustainable when population densities are low enough to allow for long fallow periods. Long fallows allow for vegetation to build up, providing large quantities of biomass for burning and the ash deposits and nutrients from mineralized organic matter are considered sufficient for reasonable crop yields in the initial year after clearing vegetation (Kyuma and Pairintra 1983; Roder et al. 1995a; Roder et al. 1996). In Laos, rapid population growth combined with government policies aimed at protecting forests and reducing the area under slash-and-burn systems are resulting in increased cropping intensity. In this paper, we define cropping intensity as the frequency of upland rice crops during a given period of time (farmers do not traditionally rotate their upland rice crop with other crops). Roder et al. (1997a) showed that fallow periods decreased from an average of 38 years during the 1950s to 5 years in A more recent survey in 2002 revealed that fallow periods of only two or three years (which are in line with the current government policies) were common (Troesch 2003). In such situations, soil nutrients become depleted and weeds become an increasingly greater problem (Roder et al. 1997a, b; Roder 2001). Intensifying upland rice cultivation without appropriate management has resulted in declining yields in Asia (Evenson et al. 1995), Africa (Nye and Greenland 1960; Van Reuler 1996; Becker and Johnson 2001) and South America (Sanchez 1976; Sanchez et al. 1982). The cause of declining yields is not clear but may be related to a combination of the following: a decline in soil fertility, an increase in soil compaction, and increased weed and insect pressure. Several studies from northern Thailand and Laos showed decreasing total carbon content and N availability as the fallow period declined (Roder et al. 1995a; Funakawa et al. 1997). The yield decline, however, can not be attributed to soil fertility alone. In fact, several studies have indicated that nutrient limitations were not responsible for the rapid yield decline observed (Sanchez et al. 1982; Evenson et al. 1995; Van Reuler 1996; George et al. 2002; Linquist et al. 2005). Results of other studies have documented relationships between cropping intensity and soil compaction (Kyuma and Pairintra 1983 in Thailand), Tetraneura nigriabdominalis root aphids (Van Keer et al. 2000; Van Keer 2003) and Meloidogyne graminicola Golden & Birchfield nematodes (Roder et al. 1998a). Also, it is well documented that weed pressure increases and the weed composition changes with increased cropping intensity (Roder et al. 1995b, 1997a). In fact, Becker and Johnson (2001) suggested that cropping intensification-induced yield loss appeared to be related mainly to increased weed infestation and declining soil N availability in West Africa. However, weeds are not likely to be the sole factor in reduced rice yields because even when weeds are controlled, yields decline. In addition to increased cropping intensity, water availability is another major constraint for rainfed upland rice production. Schiller et al. (2001) showed that between 1966 and 1999 drought was a problem in one third of the years. Furthermore, farmers in northern Laos mentioned that drought was one of the main problems associated with upland rice production (Roder et al. 1997a). The effects of cropping intensification and rainfall availability on upland rice productivity have not been well quantified in northern Laos or elsewhere. The objectives of this paper are to quantify the effect of rainfall on upland rice yields, and examine the effect of upland rice cropping intensification on the yields as well as soil N and P availability and root pest infestations (Table 1). Materials and methods Description of study area Data from field experiments and rainfall were collected from the Northern Agriculture and Forestry Research Center (NAFReC, N, E, 350 m asl.), Luang Prabang province. The soils are

3 Plant Soil (2006) 284: Table 1 Summary of main objectives, methodologies and number of upland rice yield records used for analysis Main objectives Methodologies n Evaluate the rainfall availability on upland rice productivity Evaluate the effect of cropping intensity effect on upland rice productivity Determine the relative importance of cropping intensity and rainfall on rice productivity Evaluate the effect of cropping intensity on soil N and P availability, nematodes and root aphids Compare yearly average rice yields to rainfall 11* Correlate upland rice yields to cropping history 54 Using multiple regression analysis, rice yield records are evaluated by cropping history and rainfall The effect of upland rice cropping intensification is evaluated on four fields with different cropping histories using a replicated experiment with plus and minus N fertilizer 46 4** * Number of year used for analysis ( ). Rainfall data were not available for 1996 and 1997 ** Number of fields investigated classified as Eutric Cambisol with a ph(h 2 O) of 6.1 and an organic carbon content of 1.6% (Roder et al. 1998b). This province has the highest proportion of upland rice cultivation in the country, with about 70% of the total rice area being used for upland rice. Average annual rainfall is 1300 mm but is erratic and about 80% of rainfall occurs during the growing season (May October). Rainfall limitations on upland rice productivity Rice yield data were collected from the control treatments of various agronomic and cultivar trials conducted at NAFReC between 1992 and 2004 (Lao- IRRI, ). To eliminate cultivar effects, only yield data for the traditional upland rice cultivars, Vieng and Dam, were used in this study as their yields and flowering time are similar (Lao-IRRI, 2000, 2001). Fallow periods varied among experiments but only data from fields with at least one year of fallow were used in the analysis. Furthermore, data were not used from experiments where there was complete crop failure (no yield) due to drought and pests. Yields from each year (n=1 10, depending on year) were combined to provide a yearly average yield (Table 2). Average yields were compared to monthly rainfall amounts collected during this same period ( ). Rainfall data were not available for 1996 and 1997 and the daily rainfall data were incomplete for the period from 1992 to 1995 (monthly data was available) so only monthly rainfall data were used for analysis. Rice was planted in hills using a dibble stick as per farmer practice with hills spaced at cm in most cases. Rice was planted from mid May to early June and harvested from late September to mid October. In all cases, rice was grown under rainfed conditions. In a few cultivar trials, 20 kg N/ha was applied after first weeding to minimize field variation. Hand weeding was carried out as necessary. Upland rice cropping intensity effect on the productivity Upland rice yield data were collected from fields with different cropping histories to quantify the effect of cropping intensity on productivity (Table 1). These data were from agronomic and cultivar trials conducted at NAFReC from 1992 to Only yield data for traditional upland rice cultivars, Vieng and Dam, were used for analysis. Included in this data set are five long-term experiments ranging from three to six years and initiated between 1992 and 2001 (Table 3, Lao-IRRI ; Roder et al. 1998a, b; Linquist et al. 2005). Cropping intensity varied from 15 years of fallow to five years of continuous annual upland rice cultivation. In order to analyze data across fields with varying cropping intensity a fallow index was developed (Table 4). The fallow index increases with decreasing cropping intensity. The fallow index is 1 15 for fields with a fallow of 1 15 years, respectively. Where there is no fallow (rice cropped in consecutive years), the fallow index ranges from 0 (for rice following a single year of rice) to 3 (for rice following 4 years of continuous rice cultivation). Importantly, the fallow index only accounts for a single cropping cycle and does not consider the period of time a field has been in that cycle. While it would have been ideal to know the longer term history of each field, this information was not available.

4 178 Plant Soil (2006) 284: Table 2 Monthly rainfall during May October (growing season) and yearly average rice yield in Northern Agriculture and Forestry Research Center, Laos in Year Rainfall (mm) Rice May June July Aug. Sep. Oct. Average rice yield (t/ha) n CV ** (%) 1992 * cd *** a ab d d ab ab ab bcd abc a ab 7 77 Mean CV (%) ** * Data for 1992 to 1995 are from Roder et al. (1998b) ** Coefficient of variance *** Values in column followed by the same letter are not significantly different at 5% level Table 3 Upland rice yields from five long-term experiments evaluating the effect of continuous upland rice cultivation versus a 1-year upland rice followed by a one-year fallow at the Northern Agriculture and Forestry Research Center, Laos. The year in parentheses is the year when the experiment was initiated Exp. 1 1 (1992) Exp. 2 (1993) Exp. 3 (1994) Exp. 4 (1995) Exp. 5 (2001) Continuous 1-yr fallow Continuous 1-yr fallow Continuous 1-yr fallow Year nd 2 nd nd 2 nd Year fallow 2.1 fallow 2.1 nd 3 fallow Year nd Year fallow 0.5 fallow fallow Year Year 6 nd 5 fallow 1 Rice yields in Exp. 1 were adapted from Roder et al. (1998b), those in Exp. 2 from Roder et al. (1998a) and Lao-IRRI ( ), those in Exp. 3 from Lao-IRRI ( ), those in Exp. 4 from Lao-IRRI ( ), and in Exp. 5 from Linquist et al. (2005) and Lao-IRRI ( ) 2 Rice was grown but the yield was not determined 3 Rice was damaged by root aphids 4 Rice germination was poor due to drought and rat damage 5 Rice was strongly damaged by white grubs and root aphids Table 4 Definition of the fallow index Fallow Index Cropping history 1 15 Indicates one to 15 years of fallow since the last upland rice crop 0 Upland rice grown for one year before (second consecutive year of rice) 1 Upland rice grown for two years before (third consecutive year of rice) 2 Upland rice grown for three years before (fourth consecutive year of rice) 3 Upland rice grown for four years before (fifth consecutive year of rice)

5 Plant Soil (2006) 284: Cropping intensification effect on soil N and P availability and existence of root aphids In 2004, the effect of upland rice cropping intensification on soil N and P availability and infestation by Tetraneura nigriabdominalis (root aphids) and Meloidogyne graminicola Golden & Birchfield (root knot nematodes) was examined in four fields with fallow indices ranging from 1 to 10 (Table 5). Slope gradients for each field ranged from 23 to 31%. In each field, two N fertilizer treatments (no N and added N fertilizer) were used to identify the effect of N application on rice yields. No other fertilizer was applied to these treatments and the soil properties of each field are presented in Table 5. Four replicates of each N treatment were laid out in a randomized complete block design with the size of each plot being 2 3 m 2. The rice cultivar, Vieng, was planted between May 13 and 18 by placing 6 10 seeds in holes made with a dibble stick (3 5 cm deep) at a spacing of cm. Nitrogen (60 kg N/ha) was applied at planting time in the form of controlledrelease urea (CRU). The N was placed in 3 5 cm deep holes separate from the rice seed. The CRU was a mix of two coated urea products (LPSS100 and LPS160: Chisso Asahi Hiryo Co. LTD., Tokyo) in the ratio of 3:7. Hand weeding was done as required. At maturity, rice yields were measured from a 2.25 m 2 area in each plot. Eight hills of rice in each plot were dug out after rice harvest to visually inspect the roots for root aphids and root knot nematode. In this study no nematode galls were observed. Soil samples (0 15 cm) were collected at the time of rice planting, 60 days after rice planting (DAP; panicle initiation), 90 DAP (flowering), and at maturity. Eight cores from non-fertilized plots of each field were pooled, air-dried, and sieved for soil analysis. Extractable P content was measured by the Bray No. 2 method (Nanjo, 1997) and soil ph in a 1:1 ratio of soil/ water extract. Total C and N contents were analyzed with a tracer mass spectrometer (Tracer MAT, Thermo Quest Co. Ltd., Tokyo). Available N content as NH 4 -N was determined by the indophenol method (Hidaka, 1997) and as NO 3 N by Griess Ilosvay method after reduction to NO 2 (Hidaka 1997). Bulk density at depths of 0 5 and 5 10 cm was measured from three randomly selected points in each field at rice planting. Statistical analysis Simple regression and multiple regression analyses were applied to assess the impact of upland rice cropping intensification, rainfall, soil N and P availability, and existence of root aphids on rice yields. Analyses of variance were conducted for combined data across four fields for rice yields in the fertilizer experiment in Results Rainfall limitation on rice productivity Annual variation in monthly rainfall was high with coefficients of variation between 26 and 75% Table 5 Soil properties of four fields with different cropping histories, and the effects of N fertilizer (N) and field (F) on rice yield at the Northern Agriculture and Forestry Research Center, Laos in 2004 Previous cropping history Soil fertility properties ph (1:1 H 2 O) Total C (g/kg) Total N (g/kg) Ext. P Avail. N Bulk density (mg/kg) (mg/kg) (g/cm 3 ) 0 5 cm* 5 10 cm* Rice yield (t/ha) Following 10-year fallow *** 3.9 Following 3-year fallow Following 1-year fallow Following 2-year rice cropping Mean No Nitrogen nitrogen (60 N kg/ha; CRU**) * Soil depth ** Controlled-release urea *** P<0.01 for field effect, P<0.01 for fertilizer effect, P<0.05 for N F interaction, LSD 0.05 (F) =0.55, LSD 0.05 (N) =0.20, LSD 0.05 (N F interaction) =0.40

6 180 Plant Soil (2006) 284: (Table 2). August, which is when the rice cultivars used in this study flower, had the highest rainfall as well as lowest annual variation in rainfall. Rainfall was low and variable in September and October; however, since this is the harvest period rainfall is not crucial. In June and July, during the vegetative and early reproductive stages, rainfall was variable. Yearly average rice yields ranged from 1.2 to 3.0 t/ha (Table 2), although there was high yield variability among fields within a given year (coefficients of variation ranged from 22 to 77%). Such variation may be due to differences in the length of the previous fallow period (all fields had at least one year of fallow), long-term cropping history or the spatial variability of soil fertility. Yields were not significantly related to total wet season rainfall (May through October) or rainfall in any single month (correlation coefficients ranged from 0.38 to 0.44 data not shown). However, yields were most closely correlated with total rainfall from June to August (r=0.63; P<0.05, Fig. 1) which corresponds to the rapid vegetative and early reproductive growth stages. If total rainfall during this period was less than 610 mm, yields averaged 1.4 t/ha, whereas if rainfall was greater than 690 mm yields averaged 2.5 t/ha. Upland rice cropping intensity effect on rice productivity In the five long-term experiments, first-year rice yields varied from 2.3 to 3.3 t/ha (first-year yields in Exp. 2 and 3 were not available) (Table 3). Yields declined with each successive year of continuous upland rice cropping. After five years of continuous upland rice cropping, fifth-year yields ranged from 0.3 to 1.0 t/ha (Exp. 2 5). While yield with continuous annual rice cropping declined in Exp. 2 from year three to five, it is not known why the yield in the third year was higher than in the second-year, but it may be due to higher rainfall in August 1995 (Table 2). Root aphids were observed in 1998 (the sixth year in Exp. 2), 2002 (the third year in Exp. 5), 2004 (the fourth year in Exp. 5), and 2005 (the fifth year in Exp. 5). For fields that had at least three previous consecutive rice crops, yields were 0.3 t/ha or less (some with no harvestable yield) when root aphids were present. By comparison, when root aphids were not present, rice yields after three consecutive rice crops averaged 1.0 t/ha. While rice yields still declined as a result of continuous cropping, the presence of root aphids appeared to exacerbate the problem. When successive one-year fallows are examined, yields also declined (Table 3 Exp 2, 3 and 5). In all cases, yields in fields with a one-year fallow where higher or equal to continuously cropped fields. Yields after a one-year fallow averaged 1.3 t/ha compared to 1.1 t/ha when rice continuously cropped. There was a positive correlation between fallow index and rice yields from the long-term experiments combined with other data collected from NAFReC (r=0.54; P<0.01, Fig. 2). This relationship is significant Fig. 1 Relationship between total rainfall in June August and the yields of upland rice at the Northern Agriculture and Forestry Research Center, Laos Rice yield (t/ha) y = Ln(x) R 2 = Rainfall (mm)

7 Plant Soil (2006) 284: y=0.86ln(x+5) R 2 =0.30 Rice yield (t/ha) Fig. 2 Relationship between upland rice yield and fallow index at the Northern Agriculture and Forestry Research Center, Laos. The fallow index increases with decreasing cropping intensity. For fallow index of 1 15 this is the number of fallow years between rice crops. A fallow index of 0 is the Fallow index second continuous year of cropping and fallow index of 1 to 3 is the third to fifth continuous rice crop, respectively. s shows all of rice yield data from the long-term experiments in Table 3, while shows rice yields from other trials. Both data sets were combined for analysis despite large variation in rice yields observed in fields with same fallow index, this is particularly the case for rice yields where the fallow index is 0 and 1 (coefficients of variation are 40 and 52%, respectively). Combined effects of cropping intensity and rainfall on rice productivity Using multiple regression analysis, 34% (multiple correlation coefficient adjusted by degrees of freedom) of the total variation in rice yield could be explained by cropping intensity (fallow index) and the rainfall during the period from June to August (P<0.01; n=46; Table 1). The standardized partial regression coefficients were 0.50 and 0.25 for fallow index and rainfall, respectively, and the effects were independent, suggesting that cropping intensity had a more dominant effect on rice productivity than rainfall in this study. Upland rice cropping intensification effects on soil N and P availability and root aphids Rice yields without N fertilizer application ranged from 1.0 to 3.2 t/ha. Rice yields were highest (3.2 t/ha) in the field following a 10-year fallow and lowest (1.0 t/ha) in the field following 2 consecutive years of rice cropping (Table 5). The 10-year fallow soil had higher ph, higher total carbon, total N, and extractable P levels at planting. Soil ph and extractable P content were lowest in the field following 2-year continuous rice cropping, but total carbon, total N and available N contents of this soil were similar to those fields with 1- and 3-year fallows (Table 5). Rice yields were correlated with ph, and total carbon, available N and extractable P at rice planting (P<0.01). Available N content was highest at the time of rice planting in the 10-year fallow field, but at 60 DAP was similar to the other fields (Fig. 3). Such substantial fluctuation of available N content over the rice-growing season is consistent with other studies (i.e. Kyuma and Pairintra 1983; Roder et al. 1995a; Saito et al. 2006). The bulk density in each field was low, averaging 1.1 g/cm 3 (0 5 cm), suggesting that soil compaction was not a problem (Table 5). In the field following 2-years of continuous rice cropping, symptoms of root aphid damage, similar to those reported by Listinger et al. (1986), were observed 30 days after planting. At rice harvest, the percentage of hills infected by root aphids was 83, 51, 29, and 23% in the fields following 2-year continuous

8 182 Plant Soil (2006) 284: Fig. 3 Available N content of soils collected from four fields at rice planting, 60 days after planting, 90 days after planting and at rice maturity in Eight cores from nonfertilized plots of each field were pooled in each sampling time Available N (mg/kg) Following 10-year fallow Following 3-year fallow Following 1-year fallow Following 2-year rice cropping 0 At rice planting 60 days after planting 90 days after planting At rice maturity rice cropping, 1-year fallow, 3-year fallow, and 10-year fallow, respectively. The effect of field (cropping intensity) on root aphid infestation was significant [P<0.01, LSD 0.05 is 23.7%], and there were no significant effects of fertilizer on root aphid infestation. Combining data from these four fields with Exp. 5 (fourth year, Table 3) show a strong relationship between cropping intensity, root aphid infestation and rice yields (Fig. 4). An analysis of rice yields revealed significant N fertilizer, field, and interaction effects. Nitrogen fertilizer application increased yields only in fields with previous fallows (Table 5). The lower response to N application in the fields following 10-year fallow was due in part to lodging. Rice yield without N application in the field following 2-year continuous rice cropping was not significantly different from that in the field following 1-year fallow, but there was a large difference in response to N fertilizer application. Discussion The results of this study provide evidence that both rainfall and cropping intensity significantly affect upland rice productivity (Figs. 1, 2). Rice yields are most closely associated with the total amount of rainfall from June through August as would be expected since this period coincides with the rapid vegetative and early reproductive growth stages. While we only provide indirect evidence for the effect of rainfall on rice productivity (due to the incomplete data set with some daily rainfall data missing), Malabuyoc et al. (1993) also found that rainfall during the reproductive stage explained 38 67% of the variation of upland rice yields in the Philippines. It must also be recognized that soil texture and topography play a crucial role in determining plant water availability (Gupta and O Toole 1986) and these factors were not directly addressed in this study. Declining upland rice yields with increasing cropping intensity is a finding that is consistent with reports from northern Laos based on farm surveys (Asian Development Bank 2001; Troesch 2003) and from the field experiments in other regions (Nye and Greenland 1960; Sanchez 1976; Sanchez et al. 1982; Evensen et al. 1995; Van Reuler 1996; Becker and Johnson, 2001). While significant effects of cropping intensification on rice yields were observed, there was high yield variation among the fields within the same fallow index especially where the fallow index was 0 (upland rice grown for one year before) or 1 (one year fallow) (Fig. 2). This may be due to the inherent heterogeneity of soil under slash-and-burn systems (Roder 2001) or the cumulative effects of previous fallow and cropping cycles. The fallow index only considers a single cycle, not the long-term cumulative effects of a system over many cropping cycles (which were not available in this study). As fallow periods get shorter (i.e. fallow index of 1) the

9 Plant Soil (2006) 284: Percentage of hills infected with root aphids (%) y = x R 2 = Following 10-year fallow Following 3-year fallow Following 1-year fallow Following 2-year rice cropping Following 3-year rice cropping Fallow index 4 3 y = x R 2 = Rice yield (t/ha) Percentage of hills infected with root aphids (%) Fig. 4 Relationships between fallow index and root aphid infestation (top) and between root aphids infestation and the yield of upland rice grown without N fertilizer (bottom) previous cropping history becomes increasingly important. For example, data from Table 3 (Exp. 2, 3 and 5) show that where there was a one-year fallow, yields after the first cycle averaged 1.8 t/ha. It is not possible to know if this is a decline in yields or not as yield data from the first year was not collected except in Exp. 5, however, such yields are close to the national average for upland rice yields (National Statistical Center 2004). After the second one-year fallow, yields declined to only 0.8 t/ha, suggesting that there are cumulative effects of short fallows which are not captured in the fallow index but need to be considered when addressing system sustainability. Clearly, a one year fallow may result in acceptable yields after one cycle but it is not sustainable in the long-term. Our studies also showed an interaction of cropping intensity with the effects of N availability on rice yields, and that as cropping intensity increases to the point where rice is grown annually in consecutive years, other factors besides soil N availability reduce yields. These findings are consistent with studies in

10 184 Plant Soil (2006) 284: Indonesia (Evensen et al. 1995) and West Africa (Van Reuler 1996). The poor response to N in fields following 2-years of continuous rice cropping may be due declining P availability (Table 5). While this is a possibility, Evensen et al. (1995), Van Reuler (1996) and George et al. (2002) showed that upland rice yields decline even when P (and N) fertilizer is added. In this study the poor productivity and response to N is most likely the result of root aphid infestation. Although information on the impact of root aphids on upland rice crop is limited (Listinger et al. 1986), a study in northern Thailand showed that the most critical factor for upland rice productivity was root aphids (Van Keer et al. 2000; Van Keer 2003). They observed that aphid infestations were more severe in the fields with continuous rice cropping than those in fields following fallow, supporting results of our study. However, root aphid problems are not always observed in continuously cropped upland rice fields. For example, there were no root aphids in 2003 even under continuous rice cropping (i.e. the third year in Exp. 5-Table 3). In years where root aphids were a problem the effects of root aphids on upland rice yields in continuously cropped fields was more severe. For example, in Exp 5 (Table 3), under continuous rice cropping yields in year 2, 4 and 5 (when there was root aphids) were 0.3 t/ha or less. In year 3, when root aphids were not observed, yields were 0.7 t/ha. It is possible that root aphids are a secondary problem where root aphids only infest stressed plants, however this remains unknown and further research is required for understanding the nature of root aphid problems. In conclusion, the cause of declining upland rice yields is a complex issue and is most likely a combination of a number of factors rather than any single factor. Certainly, further research is needed to fully understand these factors and interactions between them. However it is clear that the long-term productivity of upland rice in Laos can not be sustained with increased cropping intensity using the current management practices. Improved crop and resource management technologies are necessary for sustainable production. Acknowledgements The research was partly funded by the Swiss Agency for Development and Cooperation (SDC) and Global environment research fund, The Ministry of the Environment, Government of Japan (Project S2-3b). The authors thank anonymous reviewers for their helpful comments on the manuscript. References Asian Development Bank (2001) Participatory poverty assessment: Lao PDR. Asian Development Bank, Vientiane, Laos, p 187 Becker M, Johnson DE (2001) Cropping intensity effects on upland rice yield and sustainability in West Africa. 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