Assigning blank values on joins_query design
If the aim of the query is to join rows from the tables that match the "land_use" as well, then you need to include those columns in the join condition:

SELECT ...
FROM dbo_AIBLVIC_LUM_PORTION_500 AS alp500
    LEFT JOIN dbo_AIBLVIC_LUM_PORTION_1600 AS alp1600
        ON  alp500.AIBL = alp1600.AIBL
        AND alp500.LAND_USE_500 = alp1600.LAND_USE_1600 ;     -- added condition